Fix to MXS-54: https://mariadb.atlassian.net/browse/MXS-54

Added log messages for failed authentication attempts.

server/modules/protocol/mysql_client.c

@@ -530,6 +530,12 @@ static int gw_mysql_do_authentication(DCB *dcb, GWBUF *queue) {
 	if (auth_ret == 0) {
 		dcb->user = strdup(client_data->user);
 	}
+	else
+	{
+	    skygw_log_write(LOGFILE_ERROR,
+		     "%s: login attempt for user '%s', authentication failed.",
+		     dcb->service->name, username);
+	}
 
 	/* let's free the auth_token now */
 	if (auth_token) {