Added settings for control HTTP code of errored form

[ZuluPro]

It could be useful ton control easily the HTTP code sent when the comment form is badly filled.

The app actually response a 200 (OK) which can sound bad for many devs. Some would like 400 or 422.

[claudep]

Thanks for the proposal but I don't think it's a good idea. When a form has an error, it should simply be redisplayed with the error messages, that's how forms are working most of the time. Hopefully, when issue #82 will be done, you should be able to subclass the comment post view and more easily customize the behavior if you need so.