ConnectionPool.Worker: do not stop with an error when there is a timeout

This produced error log messages about GenServer termination
every time the connection was not open due to a timeout.

Instead we stop with `{:shutdown, <gun_error>}` since shutting down
when the connection can't be established is normal behavior.

lib/pleroma/gun/connection_pool.ex

@@ -53,7 +53,10 @@ defp get_gun_pid_from_worker(worker_pid, register) do
         {:ok, pid}
 
       {:DOWN, ^ref, :process, ^worker_pid, reason} ->
-        {:error, reason}
+        case reason do
+          {:shutdown, error} -> error
+          _ -> {:error, reason}
+        end
     end
   end
 

lib/pleroma/gun/connection_pool/worker.ex

@@ -31,7 +31,7 @@ def handle_continue({:connect, [key, uri, opts, client_pid]}, _) do
        :hibernate}
     else
       err ->
-        {:stop, err, nil}
+        {:stop, {:shutdown, err}, nil}
     end
   end