Fixes in GTFS-realtime export

Fixes problem with GTFS-realtime export of service alerts which lack time periods or affected entities as described by @barbeau in [this message](https://groups.google.com/d/topic/onebusaway-developers/viMd7-TeVzA/discussion).

Also fixes a problem where errors produced (such as an invalid API key) would result in a nonspecific exception being thrown, rather than a human-readable error message (as is the case for the XML and JSON APIs), as described [here](https://github.com/OneBusAway/onebusaway-application-modules/pull/52#issuecomment-19502804).