Use ABCMeta for storm.exceptions?

In order to make error handling easier in storm.django, I filed https://code.djangoproject.com/ticket/15901 about getting Django to stop catching and re-raising errors from the database adapter so we could see them properly.

I had suggested a scheme similar to what we use with the Storm exceptions where we patch them into the inheritance hierarchy of the database adapter's existing exceptions. This didn't go over too well, and I had a second idea on how to achieve this using the abc module added in Python 2.6.

It lets us achieve a similar effect without patching __bases__. For instance:

    import abc

    class MyException(Exception):
        __metaclass__ = abc.ABCMeta

    MyException.register(ZeroDivisionError)

    try:
        1/0
    except MyException:
        print "Caught ZeroDivisionError with MyException"

Perhaps it would be cleaner if we did the same sort of thing in Storm?

It currently has some problems with Python 3.2 (see http://bugs.python.org/issue12029), but it seems that they consider this a bug to be fixed though.