Fix bug when launching a game with no main scene

Check for a main scene after loading project settings and exit if there's none (except if launching in editor mode).

main/main.cpp

@@ -743,6 +743,15 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 		OS::get_singleton()->print("Error: Could not load game path '%s'.\n", game_path.ascii().get_data());
 
 		goto error;
+#endif
+	} else if (String(GLOBAL_DEF("application/run/main_scene", "")) == "") {
+#ifdef TOOLS_ENABLED
+		if (!editor) {
+#endif
+			OS::get_singleton()->print("Error: Can't run project: no main scene defined.\n");
+			goto error;
+#ifdef TOOLS_ENABLED
+		}
 #endif
 	}