Fix for failure edge case in TwitterBridge outgoing repeat/retweets.

Fix for failure edge case in TwitterBridge outgoing repeat/retweets.

When the retweet failed with a 403 error (say due to it being a private tweet, which can't be retweeted) we would end up mishandling the return value from our internal error handling.
Instead of correctly discarding the message and closing out the queue item, we ended up trying to save a bogus twitter<->local ID mapping, which threw another exception and lead the queue system to re-run it.

- Fixed the logic check and return values for the retweet case in broadcast_twitter().
- Added doc comments explaining the return values on some functions in twitter.php
- Added check on Notice_to_status::saveNew() for empty input -- throw an exception before we try to actually insert into db. :)