Merge successResultOf-traceback-6377: more useful traceback if failure

Author: cyli
Reviewer: tom.prince
Fixes: #6377

If the Deferred provided to successResultOf errbacks, print a traceback instead of just the repr of the failure.


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@37828 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb