Upgrade logs (#1495)

## Issue Addressed

#1483 

## Proposed Changes

Upgrades the log to a critical if a listener fails. We are able to listen on many interfaces so a single instance is not critical. We should however gracefully shutdown the client if we have no listeners, although the client can still function solely on outgoing connections.

For now a critical is raised and I leave #1494 for more sophisticated handling of this. 

This also updates discv5 to handle errors of binding to a UDP socket such that lighthouse is now able to handle them.

beacon_node/eth2_libp2p/src/discovery/mod.rs

@@ -210,7 +210,7 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
 
         // Start the discv5 service and obtain an event stream
         let event_stream = if !config.disable_discovery {
-            discv5.start(listen_socket);
+            discv5.start(listen_socket).map_err(|e| e.to_string())?;
             debug!(log, "Discovery service started");
             EventStream::Awaiting(Box::pin(discv5.event_stream()))
         } else {