## Issue Addressed
NA
## Proposed Changes
Return a very specific error when at attestation reads shuffling from a frozen `BeaconState`. Previously, this was returning `MissingBeaconState` which indicates a much more serious issue.
## Additional Info
Since `get_inconsistent_state_for_attestation_verification_only` is only called once in `BeaconChain::with_committee_cache`, it is quite easy to reason about the impact of this change.