Improve handling of misconfigured LDAP accounts.

Created by: patthoyts

Gitlab requires an email address for all user accounts as this is the default account id and is used for sending notifications. LDAP accounts may be missing email fields so handle this by showing a sensible error message before redirecting to the login screen again.

Resolves github issue #899 (closed)

Signed-off-by: Pat Thoyts patthoyts@users.sourceforge.net