Don't try to call library functions during arc sanity checking

Summary: These functions won't exist yet. See https://github.com/phacility/arcanist/commit/7e2df9a5bfc635b77c6412ef5a6569edd0e4e138#commitcomment-9172274

Test Plan: Faked a fatal, ran arc, got message.

Reviewers: joshuaspence, chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11254