Sync error consider adding an alarm

[tsywkGo]

for {
		needFlush := false
		needSavePos := false

		select {
		case v := <-r.syncCh:
			switch v := v.(type) {
			case posSaver:
				now := time.Now()
				if v.force || now.Sub(lastSavedTime) > 3*time.Second {
					lastSavedTime = now
					needFlush = true
					needSavePos = true
					pos = v.pos
				}
			case []*elastic.BulkRequest:
				reqs = append(reqs, v...)
				needFlush = len(reqs) >= bulkSize
			}
		case <-ticker.C:
			needFlush = true
		case <-r.ctx.Done():
			return
		}

		if needFlush {
			// TODO: retry some times?
			if err := r.doBulk(reqs); err != nil {
				log.Errorf("do ES bulk err %v, close sync", err)
				r.cancel()
				return
			}
			reqs = reqs[0:0]
		}

		if needSavePos {
			if err := r.master.Save(pos); err != nil {
				log.Errorf("save sync position %s err %v, close sync", pos, err)
				r.cancel()
				return
			}
		}
	}

When an error occurs, exit directly, r.syncCh will be blocked; But there is no alarm; So consider add an alarm.