Improve error message for LDAP users that can't sign in due to restricted domains setting

Zendesk: https://gitlab.zendesk.com/agent/tickets/30197

LDAP users get a confusing error message when they are blocked from signing in due to the restricted domains configuration. The error is Could not authenticate you from Ldapmain because "Undefined method provider for nil:nilclass".

We should better detect what the actual error is and surface that instead.