Raise an exception if LDAP search fails

Summary: Currently, we assume search will always succeed if bind succeeds, but this isn't true in the wild. We don't have a root cause yet, but blunt this error into something vaguely useful rather than passing null to an array typehint and failing later and less usefully.

Test Plan: Used bin/auth ldap.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7679