Just shows a config error page if the bouncer is not enabled. Uses a new
endpoint for this so that it can work nicely for both browser and
desktop app clients.
It's necessary, because the desktop app expects to get a json response
with either an error or billing_access_url to redirect to. Showing a
nice config error page can't be done via the json error mechanism, so
instead we just serve a redirect to the new error page, which the app
will open in the browser in a new window or tab.
e39f400f94