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.

beacon_node/beacon_chain/src/lib.rs

@@ -9,7 +9,7 @@ mod canonical_head;
 mod checkpoint;
 pub mod dump;
 mod finalized_head;
-mod info;
+mod getters;
 mod lmd_ghost;
 mod state;