Have html_error_template use pygments if available

**Migrated issue, originally created by Philip Jenvey ([@pjenvey](https://github.com/pjenvey))**

If the error template can import pygments, it should syntax highlight the HTML version of the traceback

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Attachments: [pygments_highlighting.diff](../wiki/imported_issue_attachments/95/pygments_highlighting.diff)

