Refactor OS exit code to be EXIT_SUCCESS by default

- `Main::setup` early exits (failure or `--help`/`--version`) now
  consistently return `EXIT_FAILURE` or `EXIT_SUCCESS` on all platforms,
  instead of 255 on some and a Godot Error code on others.
- `Main::start` now returns the exit code, simplifying the handling of early
  failures.
- `Main::iteration` needs to explicit set the exit code in OS if it errors
  out.
- Web and iOS now properly return `OS::get_exit_code()` instead of 0.

core/os/os.h

@@ -56,7 +56,8 @@ class OS {
 	bool _verbose_stdout = false;
 	bool _debug_stdout = false;
 	String _local_clipboard;
-	int _exit_code = EXIT_FAILURE; // unexpected exit is marked as failure
+	// Assume success by default, all failure cases need to set EXIT_FAILURE explicitly.
+	int _exit_code = EXIT_SUCCESS;
 	bool _allow_hidpi = false;
 	bool _allow_layered = false;
 	bool _stdout_enabled = true;