Improve exception handling in scalac

[jvican]

As put by @retronym in scala/scala#6067:

I'm guessing that partest is culpable for the old favourite, println(throwable.getMessage). But I'm not 100% sure, we've got similarly horrifying exception handling in the compiler itself.
A quick fix to diagnose this would be to add a commit that changes Run.compileInternal to catch Throwable and print the results to system out. That should show up in the partest log.

I've run into this issue while fixing issues in the typechecker lately. I think it would be handy to improve this soon. What about adding the stack trace information to the error handling of both partest and compileInternal?

[jvican]

I'll be sending a PR for this after 2.12.4. Suggestions are welcome.