Raise 4xx and 5xx errors instead of RTSP::Error

[turboladen]

All RTSP errors currently result in a RTSP::Error. Look to some other common Ruby libraries (net-http, savon, rest-client, rack, etc) for how they handle this case, pick one, and follow suit.

Also, be sure to let clients turn off these raises--perhaps they won't want to end execution of their app on an error.