code: function addOption(optName, optDefault) {
    if (typeof optDefault !== 'function') {
      const val = optDefault;
      optDefault = () => val;
    }
    customOptions[optName] = optDefault;
  }
docs: Adds custom options to errors' body.
  @param  {string} optName Name of the option key to add.
  @param  {}       [optDefault] Default value for the option.
  If a function is provided it will be called with
  (errorCode, errorHttpCode, errorMessage) as parameters.
  You can use this behaviour to provide different default for each error.