SF bug #1014215:  Unspecific errors with metaclass

High level error message was stomping useful detailed messages from lower
level routines.

The new approach is to augment string error messages returned by the low
level routines.  The provides both high and low level information.  If
the exception value is not a string, no changes are made.

To see the improved messages in action, type:
   import random
   class R(random): pass
   class B(bool): pass
1 file changed