LDAP connection error handling

As infra/documentation#157 suggests, LDAP connection errors (e.g. caused by a restart of the LDAP server) cause users to be falsly unsubscribed.

marked this issue as related to django-auth-ldap-remoteuser#1 (closed)

assigned to @julian

Same cause as in django-auth-ldap-remoteuser#1 (closed). Falsely unsubscribing users loses the user's settings for the list, including the mail address (if the user has more than one).

closed with commit bc3b1879