Improve logging for syntax highlighting parsing

Summary:
add logging when syntax highlighting parsing throws exception.

Test Plan:
test when exception is thrown with non-php code. I couldn't
create a file to trigger an exception in running pygmentiza, so I
manually threw an exception to test it.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, epriestley
CC: aran, tuomaspelkonen, jungejason
Differential Revision: 350