Improve the way error.php works, or how error is displayed in tool_registrationrules_pre_signup_requests()

[daledavies]

It feels like there should be a nicer way to do this

[phager-at]

Instead of redirecting the user, we could also render our own page directly in our callback and exit from all PHP execution instead. We'd even be able to customise the HTTP Status code returned to an appropriate one (like 403).