Back in 8047340d basic support was added in
the VM to handle return statements within a finally block.  But it didn't
cover all cases, in particular when some finally's were active and others
inactive when the "return" was executed.

This patch adds further support for return-within-finally by correctly
managing the currently_in_except_block flag, and should fix all cases.  The
main point is that finally handlers remain on the exception stack even if
they are active (currently being executed), and the unwind return code
should only execute those finally's which are inactive.

New tests are added for the cases which now pass.