Fix error in the error handler when the error is null
Fix error in the error handler when the error is null

mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/screenshots,fzzzy/pageshot,fzzzy/pageshot,mozilla-services/screenshots,mozilla-services/pageshot,fzzzy/pageshot,mozilla-services/screenshots,mozilla-services/screenshots