Make separate errors for epoch cache.

Helps with troubleshooting.

eth2/types/src/beacon_state.rs

@@ -1,13 +1,12 @@
-use self::epoch_cache::EpochCache;
+use self::epoch_cache::{EpochCache, Error as EpochCacheError};
 use crate::test_utils::TestRandom;
 use crate::*;
 use int_to_bytes::int_to_bytes32;
 use pubkey_cache::PubkeyCache;
 use rand::RngCore;
 use serde_derive::{Deserialize, Serialize};
-use ssz::{hash, ssz_encode, SignedRoot, TreeHash};
+use ssz::{hash, ssz_encode, TreeHash};
 use ssz_derive::{Decode, Encode, TreeHash};
-use std::collections::HashMap;
 use test_random_derive::TestRandom;
 
 mod epoch_cache;
@@ -44,6 +43,7 @@ pub enum Error {
         registry_len: usize,
     },
     RelativeEpochError(RelativeEpochError),
+    EpochCacheError(EpochCacheError),
 }
 
 macro_rules! safe_add_assign {
@@ -883,3 +883,9 @@ impl From<RelativeEpochError> for Error {
         Error::RelativeEpochError(e)
     }
 }
+
+impl From<EpochCacheError> for Error {
+    fn from(e: EpochCacheError) -> Error {
+        Error::EpochCacheError(e)
+    }
+}

eth2/types/src/beacon_state/epoch_cache.rs

@@ -1,9 +1,16 @@
-use super::{BeaconState, Error};
+use super::BeaconState;
 use crate::*;
 use honey_badger_split::SplitExt;
 use serde_derive::{Deserialize, Serialize};
 use swap_or_not_shuffle::shuffle_list;
 
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    UnableToShuffle,
+    NoValidators { epoch: Epoch },
+    UnableToGenerateSeed,
+}
+
 mod tests;
 
 #[derive(Debug, Default, PartialEq, Clone, Serialize, Deserialize)]
@@ -212,7 +219,7 @@ impl EpochCrosslinkCommitteesBuilder {
         active_validator_indices: Vec<usize>,
         registry_change: bool,
         spec: &ChainSpec,
-    ) -> Result<Self, BeaconStateError> {
+    ) -> Result<Self, Error> {
         let current_epoch = state.current_epoch(spec);
         let next_epoch = state.next_epoch(spec);
         let committees_per_epoch = spec.get_epoch_committee_count(active_validator_indices.len());
@@ -221,7 +228,9 @@ impl EpochCrosslinkCommitteesBuilder {
             current_epoch - state.validator_registry_update_epoch;
 
         let (seed, shuffling_start_shard) = if registry_change {
-            let next_seed = state.generate_seed(next_epoch, spec)?;
+            let next_seed = state
+                .generate_seed(next_epoch, spec)
+                .map_err(|_| Error::UnableToGenerateSeed)?;
             (
                 next_seed,
                 (state.current_shuffling_start_shard + committees_per_epoch) % spec.shard_count,
@@ -229,7 +238,9 @@ impl EpochCrosslinkCommitteesBuilder {
         } else if (epochs_since_last_registry_update > 1)
             & epochs_since_last_registry_update.is_power_of_two()
         {
-            let next_seed = state.generate_seed(next_epoch, spec)?;
+            let next_seed = state
+                .generate_seed(next_epoch, spec)
+                .map_err(|_| Error::UnableToGenerateSeed)?;
             (next_seed, state.current_shuffling_start_shard)
         } else {
             (
@@ -247,9 +258,9 @@ impl EpochCrosslinkCommitteesBuilder {
         })
     }
 
-    pub fn build(self, spec: &ChainSpec) -> Result<EpochCrosslinkCommittees, BeaconStateError> {
+    pub fn build(self, spec: &ChainSpec) -> Result<EpochCrosslinkCommittees, Error> {
         if self.active_validator_indices.is_empty() {
-            return Err(Error::NoValidators);
+            return Err(Error::NoValidators { epoch: self.epoch });
         }
 
         let shuffled_active_validator_indices = shuffle_list(