docs: def fail(self, err='MockupDB query failure', *args, **kwargs):
          """Reply to a query with the QueryFailure flag and an '$err' key.
  
          Returns True so it is suitable as an `~MockupDB.autoresponds` handler.
          """
          kwargs.setdefault('flags', 0)
          kwargs['flags'] |= REPLY_FLAGS['QueryFailure']
          kwargs['$err'] = err
          self.replies(*args, **kwargs)
          return True