Return 403 instead of 500 error for disconnect view

[chdsbd]

When a user only has a social account associated with them and they attempt to disconnect it, we should return a 403 error, not a 500 error.

[BarnabasSzabolcs]

Hi, as a user of rest-auth, thanks for the contribution! This repo is not maintained anymore, so the development moved to dj-rest-auth. (reference: https://github.com/Tivix/django-rest-auth/issues/568) It may be best, if you move this PR there. (and upgrade to using dj_rest_auth)

new repo link: https://github.com/jazzband/dj-rest-auth (I'm not the upkeeper of that repo, it just makes sense for me to help you merge your PR)

Many Thanks, Barney