Don't save highlithing errors to cache

Summary:
If I have Pygments enabled in config but `pygmentize` doesn't work then unhighlighted source is stored to cache.
If I later make `pygmentize` work then the unhighlighted source is still loaded from the cache.

Test Plan:
Break `pygmentize`.
View a diff with JS files.
Fix `pygmenize`.
View the diff again.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, gatos99, Koolvin

Differential Revision: https://secure.phabricator.com/D2227

src/applications/differential/parser/changeset/__init__.php

@@ -25,6 +25,7 @@ phutil_require_module('phutil', 'error');
 phutil_require_module('phutil', 'events/engine');
 phutil_require_module('phutil', 'future');
 phutil_require_module('phutil', 'markup');
+phutil_require_module('phutil', 'markup/syntax/highlighter/default');
 phutil_require_module('phutil', 'utils');