Simplify exception chaining and nested exception error messages.

Fixes #43

Given a/b/c/d.zcml included in that order with an error in d.zcml,
previously we would get this:

```
Traceback (most recent call last):
   ...
    raise InvalidDottedName(value).with_field_and_value(self, value)
zope.schema.interfaces.InvalidDottedName: invalid dotted name

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
   ...
    raise InvalidDottedName(value).with_field_and_value(self, value)
zope.configuration.exceptions.ConfigurationError: ('Invalid value for', 'package', 'invalid dotted name')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
   ...
    raise InvalidDottedName(value).with_field_and_value(self, value)
zope.configuration.xmlconfig.ZopeXMLConfigurationError: File "/tmp/d.zcml", line 2.4-2.58
    ConfigurationError: ('Invalid value for', 'package', 'invalid dotted name')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
   ...
    raise InvalidDottedName(value).with_field_and_value(self, value)
zope.configuration.xmlconfig.ZopeXMLConfigurationError: File "/tmp/c.zcml", line 2.4-2.29
    ZopeXMLConfigurationError: File "/tmp/d.zcml", line 2.4-2.58
    ConfigurationError: ('Invalid value for', 'package', 'invalid dotted name')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
   ...
    raise InvalidDottedName(value).with_field_and_value(self, value)
zope.configuration.xmlconfig.ZopeXMLConfigurationError: File "/tmp/b.zcml", line 2.4-2.29
    ZopeXMLConfigurationError: File "/tmp/c.zcml", line 2.4-2.29
    ZopeXMLConfigurationError: File "/tmp/d.zcml", line 2.4-2.58
    ConfigurationError: ('Invalid value for', 'package', 'invalid dotted name')

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  ...
    raise InvalidDottedName(value).with_field_and_value(self, value)
zope.configuration.xmlconfig.ZopeXMLConfigurationError: File "/tmp/a.zcml", line 2.4-2.29
    ZopeXMLConfigurationError: File "/tmp/b.zcml", line 2.4-2.29
    ZopeXMLConfigurationError: File "/tmp/c.zcml", line 2.4-2.29
    ZopeXMLConfigurationError: File "/tmp/d.zcml", line 2.4-2.58
    ConfigurationError: ('Invalid value for', 'package', 'invalid dotted name')
```

Now we get the simpler:

```
Traceback (most recent call last):
  ...
  File "/Users/jmadden/Projects/GithubSources/zope.schema/src/zope/schema/_field.py", line 670, in _validate
    raise InvalidDottedName(value).with_field_and_value(self, value)
zope.schema.interfaces.InvalidDottedName: invalid dotted name

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  ...
    raise InvalidDottedName(value).with_field_and_value(self, value)
zope.configuration.exceptions.ConfigurationError: Invalid value for 'package': InvalidDottedName('invalid dotted name')
    zope.schema.interfaces.InvalidDottedName: invalid dotted name
    File "/tmp/d.zcml", line 2.4-2.58
    File "/tmp/c.zcml", line 2.4-2.29
    File "/tmp/b.zcml", line 2.4-2.29
    File "/tmp/a.zcml", line 2.4-2.29
```

7 files changed, 124 insertions, 75 deletions

diff --git a/CHANGES.rst b/CHANGES.rst
index 93d7bef..19389a2 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -4,7 +4,8 @@ Changes
 4.2.3 (unreleased)
 ------------------
 
-- Nothing changed yet.
+- Simplify exception chaining and nested exception error messages.
+  See `issue 43 <https://github.com/zopefoundation/zope.configuration/issues/43>`_.
 
 
 4.2.2 (2018-09-27)
diff --git a/docs/narr.rst b/docs/narr.rst
index fc7d804..fa60dc4 100644
--- a/docs/narr.rst
+++ b/docs/narr.rst
@@ -1162,16 +1162,15 @@ redefine our directives:
 .. doctest::
 
    >>> from zope.configuration.xmlconfig import string
-   >>> from zope.configuration.xmlconfig import ZopeXMLConfigurationError
-   >>> try:
-   ...    v = string(
+   >>> from zope.configuration.exceptions import ConfigurationError
+   >>> v = string(
    ...      '<text xmlns="http://sample.namespaces.zope.org/schema" name="x" />',
    ...      context)
-   ... except ZopeXMLConfigurationError as e:
-   ...   v = e
-   >>> print(v)
-   File "<string>", line 1.0
-       ConfigurationError: The directive ('http://sample.namespaces.zope.org/schema', 'text') cannot be used in this context
+   Traceback (most recent call last):
+   ...
+   zope.configuration.exceptions.ConfigurationError: The directive ('http://sample.namespaces.zope.org/schema', 'text') cannot be used in this context
+     File "<string>", line 1.0
+
 
 Let's see what happens if we declare duplicate fields:
 
@@ -1187,7 +1186,7 @@ Let's see what happens if we declare duplicate fields:
    ...      </schema>
    ...      ''',
    ...      context)
-   ... except ZopeXMLConfigurationError as e:
+   ... except ConfigurationError as e:
    ...   v = e
    >>> print(v)
    File "<string>", line 5.7-5.24
diff --git a/src/zope/configuration/config.py b/src/zope/configuration/config.py
index e7ba289..1baf5b3 100644
--- a/src/zope/configuration/config.py
+++ b/src/zope/configuration/config.py
@@ -27,6 +27,7 @@ from zope.schema import URI
 from zope.schema import ValidationError
 
 from zope.configuration.exceptions import ConfigurationError
+from zope.configuration.exceptions import ConfigurationWrapperError
 from zope.configuration.interfaces import IConfigurationContext
 from zope.configuration.interfaces import IGroupingContext
 from zope.configuration.fields import GlobalInterface
@@ -41,7 +42,6 @@ __all__ = [
     'ConfigurationContext',
     'ConfigurationAdapterRegistry',
     'ConfigurationMachine',
-    'ConfigurationExecutionError',
     'IStackItem',
     'SimpleStackItem',
     'RootStackItem',
@@ -739,17 +739,12 @@ class ConfigurationMachine(ConfigurationAdapterRegistry, ConfigurationContext):
             ...   (2, f, (2,)),
             ...   (3, bad, (), {}, (), 'oops')
             ...   ]
-            >>> try:
-            ...    v = context.execute_actions()
-            ... except ConfigurationExecutionError as e:
-            ...    v = e
-            >>> lines = str(v).splitlines()
-            >>> 'AttributeError' in lines[0]
-            True
-            >>> lines[0].endswith("'function' object has no attribute 'xxx'")
-            True
-            >>> lines[1:]
-            ['  in:', '  oops']
+
+            >>> context.execute_actions()
+            Traceback (most recent call last):
+            ...
+            zope.configuration.config.ConfigurationExecutionError: oops
+                AttributeError: 'function' object has no attribute 'xxx'
 
         Note that actions executed before the error still have an effect:
 
@@ -769,29 +764,31 @@ class ConfigurationMachine(ConfigurationAdapterRegistry, ConfigurationContext):
                 info = action['info']
                 try:
                     callable(*args, **kw)
-                except pass_through_exceptions:
-                    raise
-                except Exception:
-                    t, v, tb = sys.exc_info()
+                except BaseException as ex:
                     try:
-                        reraise(ConfigurationExecutionError(t, v, info),
-                                None, tb)
+                        if isinstance(ex, ConfigurationError):
+                            ex.append_details(info)
+                            raise
+                        if isinstance(ex, pass_through_exceptions):
+                            raise
+                        if not isinstance(ex, Exception):
+                            # BaseException
+                            raise
+
+                        # Wrap it up and raise.
+                        reraise(ConfigurationExecutionError(info, ex),
+                                None, sys.exc_info()[2])
                     finally:
-                        del t, v, tb
+                        del ex
 
         finally:
             if clear:
                 del self.actions[:]
 
-class ConfigurationExecutionError(ConfigurationError):
+class ConfigurationExecutionError(ConfigurationWrapperError):
     """
     An error occurred during execution of a configuration action
     """
-    def __init__(self, etype, evalue, info):
-        self.etype, self.evalue, self.info = etype, evalue, info
-
-    def __str__(self): # pragma: no cover
-        return "%s: %s\n  in:\n  %s" % (self.etype, self.evalue, self.info)
 
 ##############################################################################
 # Stack items
@@ -1671,7 +1668,7 @@ def toargs(context, schema, data):
             try:
                 args[str(name)] = field.fromUnicode(s)
             except ValidationError as v:
-                reraise(ConfigurationError("Invalid value for", n, str(v)),
+                reraise(ConfigurationError("Invalid value for %r: %r" % (n, v)).append_details(v),
                         None, sys.exc_info()[2])
         elif field.required:
             # if the default is valid, we can use that:
diff --git a/src/zope/configuration/exceptions.py b/src/zope/configuration/exceptions.py
index dce50a7..1262035 100644
--- a/src/zope/configuration/exceptions.py
+++ b/src/zope/configuration/exceptions.py
@@ -14,6 +14,8 @@
 """Standard configuration errors
 """
 
+import traceback
+
 __all__ = [
     'ConfigurationError',
 ]
@@ -21,3 +23,44 @@ __all__ = [
 class ConfigurationError(Exception):
     """There was an error in a configuration
     """
+
+    # A list of strings or things that can be converted to strings,
+    # added by append_details as we walk back up the call/include stack.
+    _details = ()
+
+    def append_details(self, info):
+        if isinstance(info, BaseException):
+            info = traceback.format_exception_only(type(info), info)
+            # Trim trailing newline
+            info[-1] = info[-1].rstrip()
+            self._details += tuple(info)
+        else:
+            self._details += (info,)
+        return self
+
+    def __str__(self):
+        s = super(ConfigurationError, self).__str__()
+        for i in self._details:
+            s += '\n    ' + str(i)
+        return s
+
+    def __repr__(self):
+        s = super(ConfigurationError, self).__repr__()
+        for i in self._details:
+            s += '\n    ' + repr(i)
+        return s
+
+
+class ConfigurationWrapperError(ConfigurationError):
+
+    USE_INFO_REPR = False
+
+    def __init__(self, info, exception):
+        super(ConfigurationWrapperError, self).__init__(repr(info) if self.USE_INFO_REPR else info)
+        self.append_details(exception)
+
+        # This stuff is undocumented and not used but we store
+        # for backwards compatibility
+        self.info = info
+        self.etype = type(exception)
+        self.evalue = exception
diff --git a/src/zope/configuration/tests/test_config.py b/src/zope/configuration/tests/test_config.py
index 4bd5930..ec100af 100644
--- a/src/zope/configuration/tests/test_config.py
+++ b/src/zope/configuration/tests/test_config.py
@@ -1810,7 +1810,7 @@ class Test_toargs(unittest.TestCase):
         with self.assertRaises(ConfigurationError) as exc:
             self._callFUT(context, ISchema, {'count': '-1'})
         self.assertEqual(exc.exception.args,
-                         ('Invalid value for', 'count', '(-1, 0)'))
+                         ("Invalid value for 'count': TooSmall(-1, 0)",))
 
 
 class Test_expand_action(unittest.TestCase):
diff --git a/src/zope/configuration/tests/test_xmlconfig.py b/src/zope/configuration/tests/test_xmlconfig.py
index e75a8f0..9e19b5a 100644
--- a/src/zope/configuration/tests/test_xmlconfig.py
+++ b/src/zope/configuration/tests/test_xmlconfig.py
@@ -29,6 +29,7 @@ BVALUE = u'bvalue'
 # pylint:disable=protected-access
 
 class ZopeXMLConfigurationErrorTests(unittest.TestCase):
+    maxDiff = None
 
     def _getTargetClass(self):
         from zope.configuration.xmlconfig import ZopeXMLConfigurationError
@@ -38,11 +39,16 @@ class ZopeXMLConfigurationErrorTests(unittest.TestCase):
         return self._getTargetClass()(*args, **kw)
 
     def test___str___uses_repr_of_info(self):
-        zxce = self._makeOne('info', Exception, 'value')
-        self.assertEqual(str(zxce), "'info'\n    Exception: value")
+        zxce = self._makeOne('info', Exception('value'))
+        self.assertEqual(
+            str(zxce),
+            "'info'\n    Exception: value"
+        )
+
 
 
 class ZopeSAXParseExceptionTests(unittest.TestCase):
+    maxDiff = None
 
     def _getTargetClass(self):
         from zope.configuration.xmlconfig import ZopeSAXParseException
@@ -53,11 +59,15 @@ class ZopeSAXParseExceptionTests(unittest.TestCase):
 
     def test___str___not_a_sax_error(self):
         zxce = self._makeOne(Exception('Not a SAX error'))
-        self.assertEqual(str(zxce), "Not a SAX error")
+        self.assertEqual(
+            str(zxce),
+            "Not a SAX error\n    Exception: Not a SAX error")
 
     def test___str___w_a_sax_error(self):
         zxce = self._makeOne(Exception('filename.xml:24:32:WAAA'))
-        self.assertEqual(str(zxce), 'File "filename.xml", line 24.32, WAAA')
+        self.assertEqual(
+            str(zxce),
+            'File "filename.xml", line 24.32, WAAA\n    Exception: filename.xml:24:32:WAAA')
 
 
 class ParserInfoTests(unittest.TestCase):
diff --git a/src/zope/configuration/xmlconfig.py b/src/zope/configuration/xmlconfig.py
index e979e13..68df11b 100644
--- a/src/zope/configuration/xmlconfig.py
+++ b/src/zope/configuration/xmlconfig.py
@@ -40,14 +40,13 @@ from zope.configuration.config import GroupingContextDecorator
 from zope.configuration.config import GroupingStackItem
 from zope.configuration.config import resolveConflicts
 from zope.configuration.exceptions import ConfigurationError
+from zope.configuration.exceptions import ConfigurationWrapperError
 from zope.configuration.fields import GlobalObject
 from zope.configuration.zopeconfigure import IZopeConfigure
 from zope.configuration.zopeconfigure import ZopeConfigure
 from zope.configuration._compat import reraise
 
 __all__ = [
-    'ZopeXMLConfigurationError',
-    'ZopeSAXParseException',
     'ParserInfo',
     'ConfigurationHandler',
     'processxmlfile',
@@ -70,7 +69,7 @@ ZCML_NAMESPACE = "http://namespaces.zope.org/zcml"
 ZCML_CONDITION = (ZCML_NAMESPACE, u"condition")
 
 
-class ZopeXMLConfigurationError(ConfigurationError):
+class ZopeXMLConfigurationError(ConfigurationWrapperError):
     """
     Zope XML Configuration error
 
@@ -80,41 +79,40 @@ class ZopeXMLConfigurationError(ConfigurationError):
     Example
 
         >>> from zope.configuration.xmlconfig import ZopeXMLConfigurationError
-        >>> v = ZopeXMLConfigurationError("blah", AttributeError, "xxx")
+        >>> v = ZopeXMLConfigurationError("blah", AttributeError("xxx"))
         >>> print(v)
         'blah'
             AttributeError: xxx
     """
-    def __init__(self, info, etype, evalue):
-        self.info, self.etype, self.evalue = info, etype, evalue
 
-    def __str__(self):
-        # Only use the repr of the info. This is because we expect to
-        # get a parse info and we only want the location information.
-        return "%s\n    %s: %s" % (
-            repr(self.info), self.etype.__name__, self.evalue)
+    USE_INFO_REPR = True
+
 
-class ZopeSAXParseException(ConfigurationError):
+class ZopeSAXParseException(ConfigurationWrapperError):
     """
     Sax Parser errors, reformatted in an emacs friendly way.
 
     Example
 
         >>> from zope.configuration.xmlconfig import ZopeSAXParseException
-        >>> v = ZopeSAXParseException("foo.xml:12:3:Not well formed")
+        >>> v = ZopeSAXParseException(Exception("foo.xml:12:3:Not well formed"))
         >>> print(v)
         File "foo.xml", line 12.3, Not well formed
+            Exception: foo.xml:12:3:Not well formed
     """
-    def __init__(self, v):
-        self._v = v
 
-    def __str__(self):
-        v = self._v
-        s = tuple(str(v).split(':'))
+    def __init__(self, exception):
+        s = tuple(str(exception).split(':'))
         if len(s) == 4:
-            return 'File "%s", line %s.%s, %s' % s
+            info = 'File "%s", line %s.%s, %s' % s
         else:
-            return str(v)
+            info = str(exception)
+        super(ZopeSAXParseException, self).__init__(info, exception)
+
+    @property
+    def _v(self):
+        # BWC for testing only
+        return self.evalue
 
 class ParserInfo(object):
     r"""
@@ -238,6 +236,17 @@ class ConfigurationHandler(ContentHandler):
     def characters(self, text):
         self.context.getInfo().characters(text)
 
+    def _handle_exception(self, ex, info):
+        if self.testing:
+            raise
+        if isinstance(ex, ConfigurationError):
+            ex.append_details(repr(info))
+            raise
+        else:
+            exc = ZopeXMLConfigurationError(info, ex)
+            reraise(exc,
+                    None, sys.exc_info()[2])
+
     def startElementNS(self, name, qname, attrs):
         if self.ignore_depth:
             self.ignore_depth += 1
@@ -268,13 +277,8 @@ class ConfigurationHandler(ContentHandler):
 
         try:
             self.context.begin(name, data, info)
-        except Exception:
-            if self.testing:
-                raise
-            reraise(ZopeXMLConfigurationError(info,
-                                              sys.exc_info()[0],
-                                              sys.exc_info()[1]),
-                    None, sys.exc_info()[2])
+        except Exception as ex:
+            self._handle_exception(ex, info)
 
         self.context.setInfo(info)
 
@@ -398,13 +402,8 @@ class ConfigurationHandler(ContentHandler):
 
         try:
             self.context.end()
-        except Exception:
-            if self.testing:
-                raise
-            reraise(ZopeXMLConfigurationError(info,
-                                              sys.exc_info()[0],
-                                              sys.exc_info()[1]),
-                    None, sys.exc_info()[2])
+        except Exception as ex:
+            self._handle_exception(ex, info)
 
 
 def processxmlfile(file, context, testing=False):