beefy: Tolerate pruned state on runtime API call (#5197)

While working on #5129 I noticed that after warp sync, nodes would
print:
```
2024-07-29 17:59:23.898 ERROR ⋮beefy: 🥩 Error: ConsensusReset. Restarting voter.    
```

After some debugging I found that we enter the following loop:
1. Wait for beefy pallet to be available: Pallet is detected available
directly after warp sync since we are at the tip.
2. Wait for headers from tip to beefy genesis to be available: During
this time we don't process finality notifications, since we later want
to inspect all the headers for authority set changes.
3. Gap sync finishes, route to beefy genesis is available.
4. The worker starts acting, tries to fetch beefy genesis block. It
fails, since we are acting on old finality notifications where the state
is already pruned.
5. Whole beefy subsystem is being restarted, loading the state from db
again and iterating a lot of headers.

This already happened before #5129.