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