code: function(attribute, message) {
      if (!this.has(attribute)) {
        this.errors[attribute] = [];
      }
  
      if (this.errors[attribute].indexOf(message) === -1) {
        this.errors[attribute].push(message);
      }
    }
docs: Add new error message for given attribute
  
  @param  {string} attribute
  @param  {string} message
  @return {void}