Support LDAP Failover

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/13921

A customer was under the impression that support for multiple LDAP servers meant that if one stopped responding the users would still be able to sign in via the secondary. I can see how this would appear possible.

Can we support failover? How might that look?

cc/ @rdavila @dbalexandre Do you have any idea?