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
I'll just let it blow up; I imagine that they will only be `None` if `reraise_as` is called outside of an `except` statement, which would be nonsensical anyway.