Dendrite never listens on -https-bind-address :443 -http-bind-address :80, never fails either

When these options are specified, there should be an expectation that this condition is reached:

```
time="2024-09-28T13:00:36.332861661Z" level=info msg="Starting external listener on :80"
time="2024-09-28T13:00:36.333589366Z" level=info msg="Starting external listener on :443"
```

if not then it should be fatal, and it shouldn't be doing anything else. 
