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

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

@@ -464,6 +464,7 @@ class DifferentialChangesetParser {
       'new' => $new_future,
     );
     foreach (Futures($futures) as $key => $future) {
+      try {
         switch ($key) {
           case 'old':
             $this->oldRender = $this->processHighlightedSource(
@@ -476,6 +477,10 @@ class DifferentialChangesetParser {
               $future->resolve());
             break;
         }
+      } catch (Exception $ex) {
+        phlog($ex);
+        throw $ex;
+      }
     }
 
     $this->applyIntraline(

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

@@ -18,6 +18,7 @@ phutil_require_module('phabricator', 'infrastructure/env');
 phutil_require_module('phabricator', 'infrastructure/javelin/markup');
 phutil_require_module('phabricator', 'storage/queryfx');
 
+phutil_require_module('phutil', 'error');
 phutil_require_module('phutil', 'filesystem');
 phutil_require_module('phutil', 'filesystem/tempfile');
 phutil_require_module('phutil', 'future');