initialization of class exceptions.  Specifically:

init_class_exc(): This function now returns an integer status of the
class exception initialization.  No fatal errors in this method now.
Also, use PySys_WriteStderr() when writing error messages.  When an
error occurs in this function, 0 is returned, but the partial creation
of the exception classes is not undone (this happens elsewhere).

Things that could trigger the fallback:

    - exceptions.py fails to be imported (due to syntax error, etc.)

    - one of the exception classes is missing (e.g. due to library
      version mismatch)

    - exception class can't be inserted into __builtin__'s dictionary

    - MemoryError instance can't be pre-allocated

    - some other PyErr_Occurred

newstdexception(): Changed the error message.  This is still a fatal
error because if the string based exceptions can't be created, we
really can't continue.

initerrors(): Be sure to xdecref the .exc field, which might be
non-NULL if class exceptions init was aborted.

_PyBuiltin_Init_2(): If class exception init fails, print a warning
message and reinstate the string based exceptions.