`exceptions`-based exceptions

Rather than Nothing/0/NaN etc. (the first option being way better than the others), it would be great to generalize code that may throw to the MonadThrow class from `exceptions`.

This way, functions using `throwM (e :: Exception)` would have the signature `MonadThrow m => ... -> m ( ... )`, where `m` may become `Maybe`, or `Either e`, or even `IO`, according to the calling context.