Fixes #804 Only warn once per process that we couldn't find the exports directory to clean.
Fixes #804 Only warn once per process that we couldn't find the exports directory to clean.

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