objexcept: Add mp_obj_exception_get_value() convenience function.

This gets "value" of exceptions in the sense as it's defined for
StopIteration.value (i.e. args[0] or None).

TODO: This really should be inline function.
Elaborate GeneratorExit (gen.close()) handling.

Handling of GeneratorExit is really peculiar - it subverts normal exception
propagation rules.