Add mask, mask_ to Distribution.Compat.Exception

Fixes #1178.

It would be great if someone who knew about async exceptions could double-check that re-enabling the code that @dcoutts disabled in ce2fc30351579e1b15ae0efa78f758ed0d73c79f is ok. We're not using "real" mask, just block and unblock, but I think in this circumstance there's actually no difference.