Improve Timer::start error message.

scene/main/timer.cpp

@@ -114,7 +114,7 @@ bool Timer::has_autostart() const {
 }
 
 void Timer::start(double p_time) {
-	ERR_FAIL_COND_MSG(!is_inside_tree(), "Timer was not added to the SceneTree. Either add it or set autostart to true.");
+	ERR_FAIL_COND_MSG(!is_inside_tree(), "Unable to start the timer because it's not inside the scene tree. Either add it or set autostart to true.");
 
 	if (p_time > 0) {
 		set_wait_time(p_time);