Log error message on slave session command failure

If the slave's response differs from the master and the slave sent an
error packet, log the contents of the error. This should make it obvious
as to what caused the failure.

server/modules/routing/readwritesplit/rwsplit_session_cmd.cc

@@ -149,7 +149,7 @@ void RWSplitSession::process_sescmd_response(SRWBackend& backend, GWBUF** ppPack
         {
             if (cmd == MYSQL_REPLY_ERR && m_sescmd_responses[id] != MYSQL_REPLY_ERR)
             {
-                MXS_INFO("Session command failed on slave '%s': %s",
+                MXS_WARNING("Session command failed on slave '%s': %s",
                             backend->name(), extract_error(*ppPacket).c_str());
             }