Survive mild misconfiguration of Realname Attributes in LDAP

Summary: Currently, we'll fatal on array typehint issues if this is misconfigured. Instead, we should just reject the configuration. See some discussion in IRC.

Test Plan: Used LDAP to log in.

Reviewers: btrahan, totorico

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6489