Ensure failure types contain only immutable items

Always make sure 'exc_type_names' is a tuple, when it comes from
either the 'exc_info' tuple or from **kwargs to enforce the failure
type being immutable (which it always should be).

Change-Id: Icbdd9bc67d26b25f510914d0b19df9caef400c6d
Merge "Ensure failure types contain only immutable items"