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 imagine that the use case is that in some cases you want to catch all possible exceptions and just reraise them as the same type but with a different message:

```python
try:
    something_where_1000_things_can_go_wrong()
except Exception:
    reraise_as("Something happened, we don't know what, but we're really sorry!")
```