Don't log errors on resubscription of gossip topics (#2613)

## Issue Addressed

Resolves #2555

## Proposed Changes

Don't log errors on resubscribing to topics. Also don't log errors if we are setting already set attnet/syncnet bits.

beacon_node/eth2_libp2p/src/behaviour/mod.rs

@@ -348,6 +348,8 @@ impl<TSpec: EthSpec> Behaviour<TSpec> {
     }
 
     /// Subscribes to a gossipsub topic.
+    ///
+    /// Returns `true` if the subscription was successful and `false` otherwise.
     pub fn subscribe(&mut self, topic: GossipTopic) -> bool {
         // update the network globals
         self.network_globals
@@ -358,13 +360,13 @@ impl<TSpec: EthSpec> Behaviour<TSpec> {
         let topic: Topic = topic.into();
 
         match self.gossipsub.subscribe(&topic) {
-            Err(_) => {
+            Err(e) => {
-                warn!(self.log, "Failed to subscribe to topic"; "topic" => %topic);
+                warn!(self.log, "Failed to subscribe to topic"; "topic" => %topic, "error" => ?e);
                 false
             }
-            Ok(v) => {
+            Ok(_) => {
                 debug!(self.log, "Subscribed to topic"; "topic" => %topic);
-                v
+                true
             }
         }
     }

beacon_node/eth2_libp2p/src/discovery/mod.rs

@@ -444,15 +444,13 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
                     ));
                 }
 
+                // The bitfield is already set to required value
                 if current_bitfield
                     .get(id)
                     .map_err(|_| String::from("Subnet ID out of bounds"))?
                     == value
                 {
-                    return Err(format!(
+                    return Ok(());
-                        "Subnet id: {} already in the local ENR already has value: {}",
-                        id, value
-                    ));
                 }
 
                 // set the subnet bitfield in the ENR
@@ -480,15 +478,13 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
                     ));
                 }
 
+                // The bitfield is already set to required value
                 if current_bitfield
                     .get(id)
                     .map_err(|_| String::from("Subnet ID out of bounds"))?
                     == value
                 {
-                    return Err(format!(
+                    return Ok(());
-                        "Subnet id: {} already in the local ENR already has value: {}",
-                        id, value
-                    ));
                 }
 
                 // set the subnet bitfield in the ENR