bug: `Countdown` might have a malfunction.

```ts
const countdown = new Countdown();

countdown.start(10_000)
    .then(() => console.log("Okey Dokey!"))
    .catch(() => console.log("Nooope!"));

// [...]

countdown.stop(new Error());
```

This caused the program to crash with an uncaught exception.  
This shouldn't be the case; the exception should be caught with the `.catch` statement.

Am I missing something?