improve error message when travel() is called on an AnimationNodeStateMachine when the state machine is not playing

(cherry picked from commit 0c968d603a119821f4768e1966c9691a78e716a9)

scene/animation/animation_node_state_machine.cpp

@@ -324,7 +324,7 @@ float AnimationNodeStateMachinePlayback::process(AnimationNodeStateMachine *p_st
 					// stopped, invalid state
 					String node_name = start_request;
 					start_request = StringName(); //clear start request
-					ERR_FAIL_V_MSG(0, "Can't travel to '" + node_name + "' if state machine is not playing.");
+					ERR_FAIL_V_MSG(0, "Can't travel to '" + node_name + "' if state machine is not playing. Maybe you need to enable Autoplay on Load for one of the nodes in your state machine or call .start() first?");
 				}
 			} else {
 				if (!_travel(p_state_machine, start_request)) {