Exceptions from tests kill mutations

I found this problem testing some threaded code which used
Thread.abort_on_exception = true. Mutations would trigger an error in
the thread an abort happened. These did not count as kills. This change
captures any errors and counts them as kills. After all, if a mutation
results in an exception then it did break something and it should be
killed.

Unfortunately, we cannot rescue a given error class so a catch all has
to do. This creates problems in testing & developing mutant itself
because it may create false positives.