Fix/catch non fatal

[odersky]

Catch NonFatal(ex) instead of Throwable. The change exhibited the problem in #226 that UnApply was not handled. This is now also fixed. review by @olhotak

Closes #226

[olhotak]

This helps with #226 partially, but if I put an empty ReTyper after FirstTransform, every UnApply results in the error: error: missing arguments for method unapply in object ...

The problem is in val unappType = typed(tree.fun): it does not supply a prototype to typed(). adaptInterpolated then calls adaptNoArgs instead of adaptToArgs, causing the error. To make it call adaptToArgs, we would need a FunProto. The logic for making an UnapplyFunProto in the other typedUnApply is quite complicated. Should we factor it out and use it in both typedUnApplys?

For testing/debugging, I've created a branch https://github.com/olhotak/dotty/tree/retyper-check that just runs a ReTyper after FirstTransform.

[odersky]

The typecheck for UnApply nodes was very much incomplete. I changed the code to be simply a hook which retypers need to override. This showed another problem: UnApply nodes in catches make it to Erasure, and then blow up with an unhandled UnApply. Reassigning to @DarkDimius for a fix.

[DarkDimius]

I'm triggering retesting on travis.