Catching and reraising exceptions properly

This new method of reraising exceptions should retain the original traceback. So if you have pdb or ipdb on, you will receive the reraised exception with a more informative message, but your traceback will actually be where the original error occurred. It will also append the original exceptions and their messages to the new message by default.

See also https://groups.google.com/forum/#!topic/pylearn-dev/UbWz5tQDH5M
That's what I'm doing here; `orig_exc_type(new_exc)` creates a new exception instance of the same type but with the message that was passed. Fred suggested adding this, but I've actually never used it, so I can remove it if needed.