cb = function(response) {
          if (self.readyState == 4) {
            try {
              var res
                , status = response.target.status.toString()
                , timeSpan = new Date() - _timestamp
                , isError = /^[45]/.test(status.slice(0, -2));
              if (!isError) {
                return;
              }
              try {
                res = JSON.parse(response.target.response);
              } catch(e) {
                res = response.target.response.substring(0, 100);
              }
              _self.format([_method, status, res, _url, timeSpan], 2);
              _self.inject();
            } catch(e){}
          }
        }
Fires xhr error event once request returns 4xx or 5xx response.

@return  void