Add HttpStatusException to control flow

[graemerocher]

Currently if you want to return a custom error status you are forced to return HttpResponse object.

We should instead add a new HttpStatusException that can be thrown.

For example:

Book save(String title) {
        if(title == null) throw new HttpStatusException(HttpStatus.UNPROCESSABLE_ENTITY, "invalid title")
}

[jameskleeh]

Assigned to Ivan