Backported from godot4: Improve NavigationServer.free() error msg when RID not found

Improves the error msg when NavigationServer.free() does not find the RID e.g. because it was already deleted or did never exist in the first place.
- smix8
b51cab3411

modules/navigation/pandemonium_navigation_server.cpp

@@ -1001,7 +1001,7 @@ COMMAND_1(free, RID, p_object) {
 
 		obstacle_owner.free(p_object);
 	} else {
-		ERR_PRINT("Invalid RID.");
+		ERR_PRINT("Attempted to free a NavigationServer RID that did not exist (or was already freed).");
 	}
 }