Code review comment for lp:~gary-lasker/software-center/display-cached-recommendations

The branch lp:~mvo/software-center/no-piston-data-means-error should fix the issue above, if that works for you as well, we should merge this. Note that I added lp:~mvo/software-center/rec-small-cleanup that also updates the TODO in _on_recommender_agent_error (among other things) to refelect that we use cached data now.