docs: def command_err(self, code=1, errmsg='MockupDB command failure',
                      *args, **kwargs):
          """Error reply to a command.
  
          Returns True so it is suitable as an `~MockupDB.autoresponds` handler.
          """
          kwargs.setdefault('ok', 0)
          kwargs['code'] = code
          kwargs['errmsg'] = errmsg
          self.replies(*args, **kwargs)
          return True