Ignore repeated errors

Only the first error for each DCB should invoke the error handler
routine. All other errors for the same DCB should be ignored.

In practice this appears to happen when epoll return two different types
of error events for the same DCB.

server/modules/protocol/MySQL/MySQLBackend/mysql_backend.c

@@ -530,7 +530,7 @@ gw_read_backend_event(DCB *dcb)
 
 static void do_handle_error(DCB *dcb, mxs_error_action_t action, const char *errmsg)
 {
-    bool succp = false;
+    bool succp = true;
     MXS_SESSION *session = dcb->session;
 
     if (!dcb->dcb_errhandle_called)