These requests cannot be submitted directly by browsers, only via
javascript, and our javascript always uses JSON format for them.
I'm fairly sure treating these as HTML requests, and showing an
error HTML page, breaks the javascript code handling the request.
Fix one case in the specs that expected a flash notice after
javascript failure, which I'm guessing didn't actually work as
desired in a browser executing javascript.
This also allows removal of some no-longer-reachable code.
711ee84c35