s3: log errors instead of stopping when checking isDirectoryAllEmpty has error

weed/s3api/s3api_objects_list_handlers.go

@@ -265,7 +265,7 @@ func (s3a *S3ApiServer) doListFilerEntries(client filer_pb.SeaweedFilerClient, d
 				} else {
 					var isEmpty bool
 					if isEmpty, err = s3a.isDirectoryAllEmpty(client, dir, entry.Name); err != nil {
-						return
+						glog.Errorf("check empty folder %s: %v", dir, err)
 					}
 					if !isEmpty {
 						eachEntryFn(dir, entry)