code: function (reason, details, cb) {
          var self = this;
          var o = {
              "taskToken": this.config.taskToken
          };
          if (reason) {
              o.reason = reason;
          }
          if (details) {
              o.details = stringify(details);
          }
  
          this.swfClient.respondActivityTaskFailed(o, function (err) {
              if (self.onDone) {
                  self.onDone();
              }
              if (cb) {
                  cb(err);
              }
  
          });
  
      }
docs: Sends a "RespondActivityTaskFailed" to AWS.
  @param {String} reason
  @param {String} details
  @param {Function} [cb] - callback