Cancelling Raven login results in 500

[rjw57]

Steps to reproduce:

1. Visit https://iar.admin.cam.ac.uk/ in a new private browsing tab
2. "Sign in with Raven"
3. Hit "Cancel"
4. Get a HTTP 500 error

The cause is that the django-ucamwebauth provider raises an exception if the user cancels login which we are not catching. The desired behaviour is that cancelling the Raven login will reject the consent request.