Prevent 500 error from occurring when someone logs in with an unapproved account

[waxlamp]

An example of a user running into this: #2164.

Instead we should intercept this and present an error message to the user, along with an offer to get in touch with the admin team.

[satra]

perhaps we should also have a forget policy after a period of time. the reason these happen is because we reject a user. of course, there are also improvements for the general user registration policy as well that we should address.