Skip deleting users with protected/restricted relations (#441)

Allow the registration profile cleanup to succeed when a user has
protected or restricted relations. The cleanup assumes the users that
are selected for cleanup have never been activated. But it is possible
for an admin to activate a user without using the registration profile.
When the user is then deactivated and the profile becomes a valid
candidate for cleanup it may have associated records which prevent it
from being deleted.

registration/models.py

@@ -281,7 +281,10 @@ def delete_expired_users(self):
                     user = profile.user
                     logger.warning('Deleting expired Registration profile {} and user {}.'.format(profile, user))
                     profile.delete()
-                    user.delete()
+                    try:
+                        user.delete()
+                    except (models.ProtectedError, models.RestrictedError) as e:
+                        logger.error('Deletion of user {} is prevented: {}'.format(user, e))
                     deleted_count += 1
             except UserModel().DoesNotExist:
                 logger.warning('Deleting expired Registration profile {}'.format(profile))