Clarify error log when registering validators (#3650)

## Issue Addressed

NA

## Proposed Changes

Adds clarification to an error log when there is an error submitting a validator registration.

There seems to be a few cases where relays return errors during validator registration, including spurious timeouts and when a validator has been very recently activated/made pending.

Changing this log helps indicate that it's "just another registration error" rather than something more serious. I didn't drop this to a `WARN` since I still have hope we can eliminate these errors completely by chatting with relays and adjusting timeouts.

## Additional Info

NA

beacon_node/http_api/src/lib.rs

@@ -2627,7 +2627,12 @@ pub fn serve<T: BeaconChainTypes>(
                     .await
                     .map(|resp| warp::reply::json(&resp))
                     .map_err(|e| {
-                        error!(log, "Error from connected relay"; "error" => ?e);
+                        error!(
+                            log,
+                            "Relay error when registering validator(s)";
+                            "num_registrations" => filtered_registration_data.len(),
+                            "error" => ?e
+                        );
                         // Forward the HTTP status code if we are able to, otherwise fall back
                         // to a server error.
                         if let eth2::Error::ServerMessage(message) = e {