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.

platform/linuxbsd/godot_linuxbsd.cpp

@@ -72,18 +72,19 @@ int main(int argc, char *argv[]) {
 	char *ret = getcwd(cwd, PATH_MAX);
 
 	Error err = Main::setup(argv[0], argc - 1, &argv[1]);
+
 	if (err != OK) {
 		free(cwd);
-
 		if (err == ERR_HELP) { // Returned by --help and --version, so success.
-			return 0;
+			return EXIT_SUCCESS;
 		}
-		return 255;
+		return EXIT_FAILURE;
 	}
 
-	if (Main::start()) {
-		os.set_exit_code(EXIT_SUCCESS);
-		os.run(); // it is actually the OS that decides how to run
+	if (Main::start() == EXIT_SUCCESS) {
+		os.run();
+	} else {
+		os.set_exit_code(EXIT_FAILURE);
 	}
 	Main::cleanup();