[hive] UI should better error message when the backend is down

If a user of Beeswax (and possibly other apps) tries to save a design while Hue is down, they get a message like "could not save design". It would be better if there was a message like "could not save design because of a lost connection to Hue. Please try again in a few moments." This will especially be helpful if we institute automatically restarting Hue if it's using too many resources.