Log slave error message on failed session command

If the master succeeds in executing a session command but the slave fails,
the error message could help explain why it failed. At the moment this is
mainly relevant for inspection of test results.

server/modules/routing/readwritesplit/rwsplit_session_cmd.cc

@@ -141,6 +141,12 @@ void RWSplitSession::process_sescmd_response(SRWBackend& backend, GWBUF** ppPack
         }
         else
         {
+            if (cmd == MYSQL_REPLY_ERR && m_sescmd_responses[id] != MYSQL_REPLY_ERR)
+            {
+                MXS_INFO("Session command failed on slave '%s': %s",
+                         backend->name(), extract_error(*ppPacket).c_str());
+            }
+
             discard_if_response_differs(backend, m_sescmd_responses[id], cmd, sescmd);
         }