Do not log slot clock error prior to genesis (#4657)

## Issue Addressed

#4654 

## Proposed Changes

Only log error if we're unable to read slot clock after genesis. 

I thought about simply down grading the `error` to a `warn`, but feel like it's still unnecessary noise before genesis, and it would be good to retain error log if we're pass genesis. But I'd be ok with just downgrading the log level, too.

Author: jimmygchen

beacon_node/network/src/subnet_service/attestation_subnets.rs

@@ -302,9 +302,16 @@ impl<T: BeaconChainTypes> AttestationService<T> {
     /// Gets the long lived subnets the node should be subscribed to during the current epoch and
     /// the remaining duration for which they remain valid.
     fn recompute_long_lived_subnets_inner(&mut self) -> Result<Duration, ()> {
-        let current_epoch = self.beacon_chain.epoch().map_err(
-            |e| error!(self.log, "Failed to get the current epoch from clock"; "err" => ?e),
-        )?;
+        let current_epoch = self.beacon_chain.epoch().map_err(|e| {
+            if !self
+                .beacon_chain
+                .slot_clock
+                .is_prior_to_genesis()
+                .unwrap_or(false)
+            {
+                error!(self.log, "Failed to get the current epoch from clock"; "err" => ?e)
+            }
+        })?;
 
         let (subnets, next_subscription_epoch) = SubnetId::compute_subnets_for_epoch::<T::EthSpec>(
             self.node_id.raw().into(),