Add optimisation for epoch processing

eth2/state_processing/src/per_epoch_processing/errors.rs

@@ -8,6 +8,11 @@ pub enum EpochProcessingError {
     NoRandaoSeed,
     PreviousTotalBalanceIsZero,
     InclusionDistanceZero,
+    /// Unable to get the inclusion distance for a validator that should have an inclusion
+    /// distance. This indicates an internal inconsistency.
+    ///
+    /// (validator_index)
+    InclusionSlotsInconsistent(usize),
     BeaconStateError(BeaconStateError),
     InclusionError(InclusionError),
 }