impl Resource Unavailable RPC error (#2072)

## Issue Addressed

Related to #1891, The error is not in the spec yet (see ethereum/eth2.0-specs#2131)

## Proposed Changes

Implement the proposed error, banning peers that send it

## Additional Info

NA

beacon_node/eth2_libp2p/src/peer_manager/mod.rs

@@ -412,6 +412,12 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
             }
             RPCError::ErrorResponse(code, _) => match code {
                 RPCResponseErrorCode::Unknown => PeerAction::HighToleranceError,
+                RPCResponseErrorCode::ResourceUnavailable => {
+                    // NOTE: This error only makes sense for the `BlocksByRange` and `BlocksByRoot`
+                    // protocols. For the time being, there is no reason why a peer should send
+                    // this error.
+                    PeerAction::Fatal
+                }
                 RPCResponseErrorCode::ServerError => PeerAction::MidToleranceError,
                 RPCResponseErrorCode::InvalidRequest => PeerAction::LowToleranceError,
                 RPCResponseErrorCode::RateLimited => match protocol {