morphologyfactory: refactor chunk bdeps exceptions

This creates a small exception hierarchy for failures to validate
stratum morphologies. This is currently a line rather than a tree,
but it will be expanded later in the patch series.

This also adds test coverage for chunk build dependencies being
omitted.

1 files changed, 13 insertions, 6 deletions

diff --git a/morphlib/morphologyfactory.py b/morphlib/morphologyfactory.py
index d0d9ea04..394d186a 100644
--- a/morphlib/morphologyfactory.py
+++ b/morphlib/morphologyfactory.py
@@ -36,6 +36,17 @@ class NotcachedError(MorphologyFactoryError):
                   "remote cache specified" % repo_name)
 
 
+class StratumError(MorphologyFactoryError):
+    pass
+
+
+class NoChunkBuildDependsError(StratumError):
+    def __init__(self, stratum, chunk):
+        StratumError.__init__(
+            self, 'No build dependencies in stratum %s for chunk %s '
+                  '(build-depends is a mandatory field)' % (stratum, chunk))
+
+
 class MorphologyFactory(object):
 
     '''An way of creating morphologies which will provide a default'''
@@ -136,14 +147,10 @@ class MorphologyFactory(object):
     def _check_and_tweak_stratum(self, morphology, reponame, sha1, filename):
         '''Check and tweak a stratum morphology.'''
 
-        for source in morphology['chunks']:  # pragma: no cover
+        for source in morphology['chunks']:
             if source.get('build-depends', None) is None:
                 name = source.get('name', source.get('repo', 'unknown'))
-                raise morphlib.Error('No build dependencies '
-                                     'stratum %s for chunk %s '
-                                     '(build-depends is a mandatory '
-                                     'field)' %
-                                     (filename, name))
+                raise NoChunkBuildDependsError(filename, name)
 
         morphology.builds_artifacts = [morphology['name']]
         morphology.needs_artifact_metadata_cached = True