NULL: Implement ERROR handling

NULL mechanism sends ERROR command rather than READY command when ZAP
handler rejects the ZAP request (status code != "200"). The body of
ERROR command contains the status code as returned by ZAP handler.

src/stream_engine.cpp

@@ -712,9 +712,6 @@ void zmq::stream_engine_t::zap_msg_available ()
 
     const int rc = mechanism->zap_msg_available ();
     if (rc == -1) {
-        //  TODO:
-        //  if (errno == EACCES)
-        //      return ERROR command to client
         error ();
         return;
     }