Fix a missed err check

sync.go

@@ -118,6 +118,9 @@ func (r *remoteServer) SendBook(book *Book, storage *FileStorage, files []*File)
 	w := multipart.NewWriter(&body)
 
 	part, err := w.CreateFormFile("meta", "meta.json")
+	if err != nil {
+		return err
+	}
 	if err := json.NewEncoder(part).Encode(book.Metadata); err != nil {
 		return err
 	}