MXS-1921: Explain why session was closed

When a client connection is closed by MaxScale before the client initiates
a controlled closing of the connection, an error message is sent. This
error message now also explains why the connection was closed to make
problem resolution easier.

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

@@ -583,6 +583,7 @@ static void do_handle_error(DCB *dcb, mxs_error_action_t action, const char *err
      */
     if (!succp)
     {
+        session->close_reason = SESSION_CLOSE_HANDLEERROR_FAILED;
         poll_fake_hangup_event(session->client_dcb);
     }
 }