Improve substream management (#3261)

## Issue Addressed

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## Proposed Changes

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## Additional Info

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beacon_node/lighthouse_network/src/peer_manager/mod.rs

@@ -457,10 +457,7 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
                 debug!(self.log, "Internal RPC Error"; "error" => %e, "peer_id" => %peer_id);
                 return;
             }
-            RPCError::HandlerRejected => {
-                // Our fault. Do nothing
-                return;
-            }
+            RPCError::HandlerRejected => PeerAction::Fatal,
             RPCError::InvalidData(_) => {
                 // Peer is not complying with the protocol. This is considered a malicious action
                 PeerAction::Fatal