Make navigator message a bit more informative

src/modules/navigator/navigator_main.cpp

@@ -338,7 +338,7 @@ Navigator::task_main()
 		if (pret == 0) {
 			/* timed out - periodic check for _task_should_exit, etc. */
 			if (global_pos_available_once) {
-				PX4_WARN("navigator timed out");
+				PX4_WARN("no GPS - navigator timed out");
 				global_pos_available_once = false;
 			}
 			continue;