s3: break loop if error

fix https://github.com/chrislusf/seaweedfs/issues/1701

weed/s3api/s3api_objects_list_handlers.go

@@ -315,7 +315,7 @@ func (s3a *S3ApiServer) isDirectoryAllEmpty(filerClient filer_pb.SeaweedFilerCli
 	currentDir := parentDir + "/" + name
 	var startFrom string
 	var isExhausted bool
-	for fileCounter == 0 && !isExhausted {
+	for fileCounter == 0 && !isExhausted && err == nil {
 		err = filer_pb.SeaweedList(filerClient, currentDir, "", func(entry *filer_pb.Entry, isLast bool) error {
 			if entry.IsDirectory {
 				subDirs = append(subDirs, entry.Name)