SAK-26151 FileUserDirectoryProvider should pass failed searches back.

When a problem occurs with one of the providers in the FilterUserDirectoryProvider it should pass that error back up to the caller. Before this it was causing a stack trace with a NPE

git-svn-id: https://source.sakaiproject.org/svn/providers/trunk@308665 66ffb92e-73f9-0310-93c1-f5514f145a0a