Check for PhutilMissingSymbolException when trying to load finfo

Summary:
If class_exists('finfo') fails, a PhutilMissingSymbolException will also be
thrown, so we should wrap a try/catch around it.

Test Plan:
Loaded a page that was throwing a PhutilMissingSymbolException because of this,
and saw that it no longer throws this execption.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: jungejason, aran, epriestley

Differential Revision: https://secure.phabricator.com/D1634