pruner + proto change: better handling of missing replication cursor

- don't treat missing replication cursor as an error in protocol
- treat it as a per-fs planning error instead

replication/fsrep/fsfsm.go

@@ -463,16 +463,11 @@ func (s *ReplicationStep) doMarkReplicated(ctx context.Context, ka *watchdog.Kee
 			},
 		},
 	}
-	res, err := sender.ReplicationCursor(ctx, req)
+	_, err := sender.ReplicationCursor(ctx, req)
 	if err != nil {
 		log.WithError(err).Error("error advancing replication cursor")
 		return err
 	}
-	if res.GetError() != "" {
-		err := fmt.Errorf("cannot advance replication cursor: %s", res.GetError())
-		log.Error(err.Error())
-		return err
-	}
 	ka.MadeProgress()
 
 	s.state = StepCompleted