Add default status 400 to error thrown by Joi.assert()

I'm currently doing this in my koa app in every route:

``` js
let object
try {
  object = Joi.attempt(context.request.body, schema)
} catch (e) {
  e.status = 400
  throw e
}
```

It would be nice for convenience to have the 400 already on the error since that is the HTTP equivalent of a validation error. I don't think there would be any downsides to this, so please consider :)
