Added a more helpful error message when there is no current animation for the animation player

(cherry picked from commit 8bfeb9d6aeca9b71904babd9071317c4ae87f7fe)

scene/animation/animation_player.cpp

@@ -1362,13 +1362,13 @@ bool AnimationPlayer::is_valid() const {
 
 float AnimationPlayer::get_current_animation_position() const {
 
-	ERR_FAIL_COND_V(!playback.current.from, 0);
+	ERR_FAIL_COND_V_MSG(!playback.current.from, 0, "AnimationPlayer has no current animation");
 	return playback.current.pos;
 }
 
 float AnimationPlayer::get_current_animation_length() const {
 
-	ERR_FAIL_COND_V(!playback.current.from, 0);
+	ERR_FAIL_COND_V_MSG(!playback.current.from, 0, "AnimationPlayer has no current animation");
 	return playback.current.from->animation->get_length();
 }