System for exceptions

[boxed]

This relates to the same discussion for Cosmic Ray: sixty-north/cosmic-ray#97

To recap my comment there: In mutmut I have a support for # pragma: no mutate that is simple and very useful and also mirrors the pragma for coverage.py closely which is nice. We're having a discussion (boxed/mutmut#47 and boxed/mutmut#28) about a more advanced system too. It would be great if we could agree on some basic specs on that too if need be. But the pragma seems like the best way for the normal cases I think.