EventLoopError::ExitFailure(code) and PumpStatus::Exit(code) shouldn't use error codes

[daxpedda]

After #3056, we should probably not use error codes anymore.

All uses of EventLoopError::ExitFailure(i32) could return EventLoopError::Os(OsError) instead. PumpStatus::Exit(i32) could probably be PumpStatus::Exit(Result<(), OsError>) instead, unless we want to add another variant for that.

[kchibisov]

Yeah, having exit code is not a good idea, it's better to propagate error and have a dedicated exit method, though, some may desire a way to exit event loop with their error somehow, or somehow indicate that loop has terminated.

[madsmtm]

But are they exiting the event loop with an error to communicate it to the operating system? Because if so, then they can just do:

let mut error_exit_code: Option<i32> = None;
event_loop.run(|event, elwt| {
    // On some error event:
    error_exit_code = Some(1);
    elwt.exit()
})?;

if let Some(exit_code) = error_exit_code {
    return exit_code;
} else {
    return 0;
}

[kchibisov]

Hm, yeah, they can sort of stash like that error, that's true and then just propagate it... Then I guess it's fine...