Add exception stack unwind support for RETURN_VALUE.

This properly implements return from try/finally block(s).

TODO: Consider if we need to do any value stack unwinding for RETURN_VALUE
case. Intuitively, this is "success" return, so value stack should be in
good shape, and unwinding shouldn't be required.
Merge pull request #251 from pfalcon/return_unwind

Add exception stack unwind support for RETURN_VALUE.