Display-side error checking in place. Paired by Michael and Richard.

The display's DatabaseReceiver now catches any URL and HTTP error raised at it because it cannot find connection. It does not automatically pick connection back up again, but the display will continue running until someone rei-establishes connection with the server. (Feature #191)
Merge pull request #91 from CO600GOL/feature/#191_display_side_error_handling

Display-side error checking in place. Paired by Michael and Richard.