code: function(e, pr) {
          if (arguments.length === 0) {
              if (this.$error !== null) {
                  this.dumpError(e);
              }
          } else {
              if (this.$error === null) {
                  if (this.$ignoreError) {
                      this.$ignored(e);
                  } else {
                      this.$taskCounter = this.$level = this.$busy = 0;
                      this.$error   = e;
                      this.$results = [];
                  }
  
                  this.$schedule();
              } else if (arguments.length < 2 || pr === true) {
                  this.dumpError(e);
              }
          }
  
          return this;
      }
docs: Force to fire error.
  @param  {Error} [e] an error to be fired
  @method error
  @chainable