Improve attester errors, move info -> helpers

- Ensured one can distingush between a committee error and an invalid
validator index when using `validator_attesation_slot_and_shard`.
- Renamed the `info.rs` file to `getters.rs`, for clarity.

eth2/types/src/beacon_state/slot_processing.rs

@@ -2,7 +2,6 @@ use crate::{
     beacon_state::{CommitteesError, EpochProcessingError},
     BeaconState, ChainSpec, Hash256,
 };
-use log::debug;
 
 #[derive(Debug, PartialEq)]
 pub enum Error {
@@ -43,17 +42,17 @@ impl BeaconState {
         &self,
         validator_index: usize,
         spec: &ChainSpec,
-    ) -> Result<(u64, u64, u64), CommitteesError> {
+    ) -> Result<Option<(u64, u64, u64)>, CommitteesError> {
         let mut result = None;
         for slot in self.get_current_epoch_boundaries(spec.epoch_length) {
             for (committee, shard) in self.get_crosslink_committees_at_slot(slot, spec)? {
                 if let Some(committee_index) = committee.iter().position(|&i| i == validator_index)
                 {
-                    result = Some(Ok((slot, shard, committee_index as u64)));
+                    result = Some((slot, shard, committee_index as u64));
                 }
             }
         }
-        result.unwrap()
+        Ok(result)
     }
 }