Allow running with a custom resource without a main scene

After 3f8a4cc7193e964f716fde2cd28a946669e2d8d6 trying to run an
individual scene on a project without a main scene fails. We move the
check until after we've determined whether or not we're trying to run an
individual scene.

We also stop trying to show the project manager if any game pack is
found at all, unless the user explicitly asks for the project manager to
be shown.

main/main.cpp

@@ -349,6 +349,7 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 	Vector<String> breakpoints;
 	bool use_custom_res = true;
 	bool force_res = false;
+	bool found_project = false;
 
 	packed_data = PackedData::get_singleton();
 	if (!packed_data)
@@ -760,7 +761,9 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 
 #endif
 
-	if (globals->setup(game_path, main_pack, upwards) != OK) {
+	if (globals->setup(game_path, main_pack, upwards) == OK) {
+		found_project = true;
+	} else {
 
 #ifdef TOOLS_ENABLED
 		editor = false;
@@ -768,15 +771,6 @@ 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
 	}
 
@@ -801,14 +795,21 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 
 	if (!project_manager) {
 		// Determine if the project manager should be requested
-		project_manager =
-				main_args.size() == 0 &&
-				!ProjectSettings::get_singleton()->has_setting("application/run/main_loop_type") &&
-				(!ProjectSettings::get_singleton()->has_setting("application/run/main_scene") ||
-						String(ProjectSettings::get_singleton()->get("application/run/main_scene")) == "");
+		project_manager = main_args.size() == 0 && !found_project;
 	}
 #endif
 
+	if (main_args.size() == 0 && String(GLOBAL_DEF("application/run/main_scene", "")) == "") {
+#ifdef TOOLS_ENABLED
+		if (!editor && !project_manager) {
+#endif
+			OS::get_singleton()->print("Error: Can't run project: no main scene defined.\n");
+			goto error;
+#ifdef TOOLS_ENABLED
+		}
+#endif
+	}
+
 	if (editor || project_manager) {
 		use_custom_res = false;
 		input_map->load_default(); //keys for editor