Deleting users can cause all kinds of inconsistencies (depending on the
Django application). The Django docs also recommends deactivating users
instead of deleting them.

Fixes #2

LDAP errors were previously misinterpreted as empty set of results, causing
mass-deletion of all user accounts. This works around the issue, but limits the
allowed type of AUTH_LDAP_USER_SEARCH_ALL_NAME to LDAPSearch.

Fixes #1