During refactoring of #5959, some implicit `return` were overlooked,
resulting in spurious `err()` being returned without message.
```
{"lvl":"WRN","ts":"2024-02-26 10:12:20.469+00:00","msg":"Beacon nodes report different configuration values","reason":"","service":"fallback_service","node":"http://127.0.0.1:9303[Nimbus/v24.2.1-4e9bc7-stateofus]","node_index":0,"node_roles":"AGBSDT"}
```
Correcting the helpers to return explicit result in all exhaustive
cases so that this cannot happen anymore by accident.
c3fe055bd4