routing: add new error for spent channel UTXOs

routing/router.go

@@ -1354,7 +1354,8 @@ func (r *ChannelRouter) processUpdate(msg interface{},
 			r.quit,
 		)
 		if err != nil {
-			return fmt.Errorf("unable to fetch utxo "+
+
+			return newErrf(ErrChannelSpent, "unable to fetch utxo "+
 				"for chan_id=%v, chan_point=%v: %v",
 				msg.ChannelID, fundingPoint, err)
 		}