All of the HTTP error codes that can be returned by the Streaming API are now documented on the Streaming API Wiki, http://apiwiki.twitter.com/Streaming-API-Documentation#HTTPResponseCodes.
---- HTTP Response Codes Most error codes are returned with a string with additional details. For all codes greater than 200, clients should wait before attempting another connection. See Connecting section, above. 200 Success 401 Unauthorized HTTP authentication failed due to either a non-existent username or an incorrect password. User authenticated properly but is not in a required role for this resource; contact the API team for appropriate access. 404 Unknown Resource does not exist. 406 Not Acceptable Parameter not allowed for resource, for example, track parameter specified on a sampled resource. Track keyword too long or too short. No predicates defined for filtered resource, for example, neither track nor follow parameter defined. Follow userid unparseable. 413 Too Long A parameter list is too long, for example, track or follow parameter string too long. Too many track tokens specified for role; contact API team for increased access. Too many follow userids specified for role; contact API team for increased access. 416 Range Unacceptable Count parameter value is too large. 500 Server Internal Error Should not occur, contact API team. 503 Service Overloaded Should not occur, contact API team.