code: function connectionCheck(error, diagnose) {
      var servers = diagnose.knownServers;
  
      // Nothing to check so we fail immediately.
      if (!servers || servers.length === 0) {
        throw new ServerDownError(error);
      }
  
      // We check all the servers that the user asked to check. If none respond,
      // we blame the network. Otherwise, we blame the server.
      return Promise.all(servers.map(function urlToAjax(url) {
        // eslint-disable-next-line no-use-before-define
        return ajax({ url: dedupURL(normalizeURL(url)), timeout: 1000 })
          .reflect();
      })).filter(function filterSuccessfulServers(result) {
        return result.isFulfilled();
      }).then(function checkAnyFullfilled(fulfilled) {
        if (fulfilled.length === 0) {
          throw new NetworkDownError(error);
        }
  
        throw new ServerDownError(error);
      });
    }
docs: This is called once we know a) the browser is not offline but b) we cannot reach the server that should serve our request.