Simplified exceptions

As of Python 2.6 (PEP 352), exceptions now support passing in string
messages as the first (and only) parameter.  These messages will be
displayed when the exception is converted to a string, so all of the
custom code we were using is no longer needed.

http://stackoverflow.com/a/6029838/359284
http://legacy.python.org/dev/peps/pep-0352/