code: function patchErrorBody(err, options) {
    // Gets the current toJSON to be extended.
    const json = err.toJSON();
  
    Object.keys(customOptions).forEach(optName => {
      let value = options[optName];
      if (value === undefined) {
        value = err.body[optName];
        if (value === '' || value === undefined) {
          value = customOptions[optName](
            err.body.code,
            err.statusCode,
            err.body.message
          );
        }
      }
      if (value !== undefined) {
        // Adds the option to the body of the error.
        err.body[optName] = value;
        // Adds the option to the json representation of the error.
        json[optName] = value;
      }
    });
  
    // Patchs the toJSON method.
    err.toJSON = () => json;
  }
docs: Monkey patch the error object after his creation.
  @param  {object} err Error to patch.
  @param {object} options Options given by the user.