Override NSError in the operation completion handler

Prevents problem with AFNetworking doing a recursive loop causing a crash.  I think further refactoring is needed to bring us current on AFNetworking 2 practices.
Merge pull request #1734 from wordpress-mobile/issue/1733-afnetworking-error-loop

Override NSError in the operation completion handler
Merge remote-tracking branch 'upstream/develop' into develop