Add a typedUnApply method.

The method is needed to be able to survive ReTypers between frontend and
pattern matching.
Replace Throwable catches with NonFatal

Previously, stackoverflows led to infinite loops because the catch
immediately threw another stack overflow. Anyway, one should never
catch Throwable.
Made typedUnApply a hook that has to be overridden in a ReTyper.

Change exhibits a problem in the transformers: pattern matching try's
do not seem to be handled.
Remove catch-rethrow in Driver
Merge pull request #285 from dotty-staging/fix/catchNonFatal

Fix/catch non fatal