Error when LDAP doesn't supply an email address

Zendesk: https://gitlab.zendesk.com/agent/tickets/16953

In this case, the customer did not have mail or any of the default mail attributes. GitLab should then assign a temporary OAuth email or ask the user to set one themselves. Instead, in this case, the log spit out Error saving user: ["Notification email is not an email you own"].

@zj Will you take a look, please?

cc/ @DouweM