Only recognize the expected output as an exception if it *starts* with

Only recognize the expected output as an exception if it *starts* with
a traceback message.  I.e., examples that raise exceptions may no
longer generate pre-exception output.  This restores the behavior of
doctest in python 2.3.  The ability to check pre-exception output is
being removed because it makes the documentation simpler; and because
there are very few use cases for it.