TwitterBridge: discard outgoing queue items instead of retrying when Twitter API...

TwitterBridge: discard outgoing queue items instead of retrying when Twitter API returns HTTP 400 or 404 error codes

We get HTTP 400 for various cases of invalid data, where retrying doesn't help at all -- previously those would loop forever, or until something died at least. :)
400 is also used for rate limiting, but retrying *immediately* will just hit the rate limit again, so better to discard if we're going over for now.