Add more error messages to monitors

When the execution of a query fails, the error reported by the Connector-C
and the server where the query was executed is logged.

include/maxscale/monitor.h

@@ -243,4 +243,11 @@ void mon_process_state_changes(MXS_MONITOR *monitor, const char *script, uint64_
  */
 void mon_hangup_failed_servers(MXS_MONITOR *monitor);
 
+/**
+ * @brief Report query errors
+ *
+ * @param db Database where the query failed
+ */
+void mon_report_query_error(MXS_MONITOR_SERVERS* db);
+
 MXS_END_DECLS