Tidy per epoch processing

- Add comments to ValidatorStatuses
- Add some checks to guard against a bad statuses list
- Remove unused attester_sets.rs file.

eth2/state_processing/src/per_epoch_processing/errors.rs

@@ -8,6 +8,7 @@ pub enum EpochProcessingError {
     NoRandaoSeed,
     PreviousTotalBalanceIsZero,
     InclusionDistanceZero,
+    ValidatorStatusesInconsistent,
     /// Unable to get the inclusion distance for a validator that should have an inclusion
     /// distance. This indicates an internal inconsistency.
     ///