another ec'

src/include/taler_error_codes.h

@@ -1993,6 +1993,13 @@ enum TALER_ErrorCode
    */
   TALER_EC_SYNC_PAYMENT_CREATE_BACKEND_ERROR = 6018,
 
+  /**
+   * The sync service failed find the backup to be updated in its database.
+   * This response is provided with HTTP status code
+   * MHD_HTTP_NOT_FOUND.
+   */
+  TALER_EC_SYNC_PREVIOUS_BACKUP_UNKNOWN = 6019,
+
   /**
    * End of error code range.
    */