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.

main/main.h

@@ -78,7 +78,7 @@ public:
 	static Error test_setup();
 	static void test_cleanup();
 #endif
-	static bool start();
+	static int start();
 
 	static bool iteration();
 	static void force_redraw();