JOI custom error message.

https://stackoverfhttps://stackoverflow.com/questions/48720942/node-js-joi-how-to-display-a-custom-error-messageslow.com/questions/48720942/node-js-joi-how-to-display-a-custom-error-messages

challengeDescription: Joi.string()
    .required()
    .error(() => {
      return {
        message: 'Challenge description is required.',
      };
    }),