Following from the conversation in #4040, I've added the default validation error to err.data.defaultError where err is the detailed error passed to failAction() for input validation. Since the common use-case seems to be logging detailed validation errors without modifying the error response, I considered logging the detailed error by default. But I determined that it contained user input which could be sensitive: I believe users should opt-in to this behavior with a custom failAction() rather than it become the default 'log' behavior. Here's how that would look with this update:

server.route({
    method: 'get',
    path: '/',
    handler: () => 'ok',
    options: {
        validate: {
            query: {
                a: Joi.string().min(2)
            },
            failAction: function (request, h, err) {
                request.log(['validation', 'error', err.payload.validation.source], err);
                throw err.data.defaultError;
            }
        }
    }
});

Resolves #4040