message.

an exception from errno, with a supplied filename (primarily used by
IOError and OSError).  If class exceptions are used then the exception
is instantiated with a 3-tuple: (errno, strerror, filename).  For
backwards compatibility reasons, if string exceptions are used,
filename is ignored.

PyErr_SetFromErrno(): Implement in terms of
PyErr_SetFromErrnoWithFilename().

by Marc Lemburg.  There's a path through the code where *val is NULL,
but value isn't, and value should be DECREF'ed.

instance's class is a subclass of this, then use the instance's class
as the exception type.

(PyExc_MemoryErrorInst) raise this instead of PyExc_MemoryError.  This
only happens when exception classes are enabled (e.g. when Python is
started with -X).

- int PyErr_GivenExceptionMatches(obj1, obj2)

  Returns 1 if obj1 and obj2 are the same object, or if obj1 is an
  instance of type obj2, or of a class derived from obj2

- int PyErr_ExceptionMatches(obj)

  Higher level wrapper around PyErr_GivenExceptionMatches() which uses
  PyErr_Occurred() as obj1.  This will be the more commonly called
  function.

- void PyErr_NormalizeException(typeptr, valptr, tbptr)

  Normalizes exceptions, and places the normalized values in the
  arguments.  If type is not a class, this does nothing.  If type is a
  class, then it makes sure that value is an instance of the class by:

  1. if instance is of the type, or a class derived from type, it does
     nothing.

  2. otherwise it instantiates the class, using the value as an
     argument.  If value is None, it uses an empty arg tuple, and if
     the value is a tuple, it uses just that.