py/runtime: Add mp_load_method_protected helper which catches exceptions

This new helper function acts like mp_load_method_maybe but is wrapped in
an NLR handler so it can catch exceptions.  It prevents AttributeError from
propagating out, and optionally all other exceptions.  This helper can be
used to fully implement hasattr (see follow-up commit), and also for cases
where mp_load_method_maybe is used but it must now raise an exception.