MXS-1950: Log error on failed COM_CHANGE_USER

If a client is executing a COM_CHANGE_USER command and the
reauthentication of the client fails, no error message would be logged
about the failure of the reauthentication process and only a routing
failure message would be logged.

server/modules/protocol/MySQL/mariadbclient/mysql_client.cc

@@ -1716,6 +1716,9 @@ static int route_by_statement(MXS_SESSION* session, uint64_t capabilities, GWBUF
                     rc = 0;
                     gwbuf_free(packetbuf);
                     packetbuf = NULL;
+                    MXS_ERROR("User reauthentication failed for '%s'@'%s'",
+                              session->client_dcb->user,
+                              session->client_dcb->remote);
                 }
             }