chore: handle error when admin attempts to delete a member with an active checkin

[kimadactyl]

Overview Deleting a member with an active checkin results in the following:

Deleting the member 'g' would result in deleting related objects, but your account doesn't have permission to delete the following types of objects:

    checkin

This should be clearer to non-admins

Acceptance Criteria

• [ ] Error changed to something more human: "You can't delete that member because they are currently checked in"
• [ ] Admins can't delete users that are checked in

Questions/Assumptions

• [ ] Are there error states that might lead to this?

[wheresalice]

I don't believe there is a way of changing this. If we were to allow admins to delete member checkins then we wouldn't have this problem.

[kimadactyl]

Hmm. OK, let's go for that route then. It does seem a pretty severe limitation of Django admin to not be able to handle error messages tho?