Node fails to start if DKG DB has state but no group file

[CluEleSsUK]

If you accidentally the group file, but maintain DB state, the node entirely fails to start.
On one hand, this catches inconsistencies immediately. On the other hand, it could deny service to other beacons if one of them gets messed up.

In the short term, we should just fail to start the single beacon that's missing the group file.
In the medium term, we can consider rebuilding the group file from the DKG DB state. Right now, public polynomials would be missing, but we could perhaps store them in the DKG state.
In the longer term, we could consider getting rid of the group file altogether and refactor the BeaconProcess to load DKG state directly from the DB.