- `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.