(TK-64) Fix TK failures from Slingshot Exceptions

Fix IllegalArgumentException that occurs when catching a
Slingshot exception in the TK main function by adding an
else clause to the case statement.
I agree that they're certainly not user-friendly.  However ... I'm not sure that's too important here.  And, in fact, just printing the stacktrace could be hiding data (not logging the entire object thrown by `throw+`), right?