update logs for listener.

pkg/postage/listener/listener.go

@@ -192,6 +192,7 @@ func (l *listener) Listen(from uint64, updater postage.EventUpdater) <-chan stru
 			l.metrics.BackendCalls.Inc()
 			to, err := l.ev.BlockNumber(ctx)
 			if err != nil {
+				l.logger.Errorf("BlockNumber failed, err=%s", err.Error())
 				l.metrics.BackendErrors.Inc()
 				return err
 			}
@@ -247,6 +248,7 @@ func (l *listener) Listen(from uint64, updater postage.EventUpdater) <-chan stru
 
 			err = updater.UpdateBlockNumber(to)
 			if err != nil {
+				l.logger.Errorf("UpdateBlockNumber again failed, err=%s", err.Error())
 				return err
 			}