[Bug] no clean way to exit application in post_init

[Poolitzer]

We have four ways to exit application, none of them work without an error. We should fix at least one and document which works:

• app.stop() returns the error that the application is not running,
• app.shutdown() gives "This Updater was not initialized",
• app.stop_running() returns because the application is not running,
• sys.exit gives "coroutine 'Updater.start_polling' was never awaited."