fix get attesting indices (#5742)

* fix get attesting indices

* better errors

* fix compile

* only get committee index once

consensus/types/src/beacon_state.rs

@@ -159,7 +159,8 @@ pub enum Error {
     IndexNotSupported(usize),
     InvalidFlagIndex(usize),
     MerkleTreeError(merkle_proof::MerkleTreeError),
-    NoCommitteeFound,
+    NoCommitteeFound(CommitteeIndex),
+    InvalidCommitteeIndex(CommitteeIndex),
 }
 
 /// Control whether an epoch-indexed field can be indexed at the next epoch or not.