Set appropriate status codes for form confirmation pages.

We've recently had incidents where form submissions were failing due to a database error in one case, and problems with the eway payment provider in another.

Form submissions in these cases returned a 302 redirect to a page that returned a 200 status code, even in case of failure. Had the status codes been set to indicate an error, our monitoring would have alerted us to the situation earlier, and it hit during a peak donation period.

I suspect it might be best for the error status to be returned for the POST request rather than for the (negative) confirmation. Given a failure though, I figure that it might be a good thing for a page reload to re-run the request.