Surface a better warning when the LDAP extension is not installed

Summary:
Fixes T3347. We can't really do this one as a config thing since we don't know if the user wants to use LDAP.

Instead, just give them a better message than they otherwise get when they try to install/configure/use LDAP.

Test Plan: Faked it and got a reasonable message.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3347

Differential Revision: https://secure.phabricator.com/D10260