Merge failureResultOf-optional-types-6380: expected types of failureResultOf

Author: cyli
Reviewer: tom.prince
Fixes: #6380

Allow failureResultOf to take optional expected failure types, so that if an unexpected failure occurs, the failureResultOf assertion will fail.


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@37834 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb