Avoid a spurious error on ShotContext teardown
Avoid a spurious error on ShotContext teardown

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