beefy: error logs for validators with dummy keys (#3939)

This outputs:
```
2024-04-02 14:36:02.135 ERROR tokio-runtime-worker beefy: 🥩 for session starting at block 21990151
no BEEFY authority key found in store, you must generate valid session keys
(https://wiki.polkadot.network/docs/maintain-guides-how-to-validate-polkadot#generating-the-session-keys)
```
error log entry, once every session, for nodes running with
`Role::Authority` that have no public BEEFY key in their keystore

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Co-authored-by: Bastian Köcher <git@kchr.de>