Feature/improve feed upload feedback

This is to fix #265.  If an error is encountered parsing GTFS, it will return the error message as JSON.  Also, if the celery feed import task gets a spray response that is not JSON, it will fall back to reading it as a text string.

It would probably be best to figure out why `OpenTransitGeoTrellisExceptionHandler` in `OpenTransitServiceActor` isn't actually catching errors from `ingestRoute`; it's not apparent to me why that's happening.