bugfix: fix reverseMap not throwing NoSuchPrincipalExceptions

Instead of throwing a NoSuchPrincipalException on failed reverse
mappings the Ldap plugin would just return an empty set.
This fixed this misbehaviour.

Ticket: http://rt.dcache.org/Ticket/Display.html?id=8422
Acked-by: Paul
Target: 2.6
Require-book: no
Require-notes: yes