Fix issue 829 -- error message is displaying wrong incorrect result.
Fix run_with_stack_context test.

This test would never fail because it didn't check the result of its
Future.  Thanks to Florian Ludwig for pointing this out.
Merge pull request #831 from erkyrath/master

Fixed incorrect error message in _when_complete (#829)
Update twisted test blacklist for the newly-released 13.1.0.