Place wrapper around comment as diff to catch panics (#15085)

Place wrapper around comment as diff to catch panics (#15085)

There are a few recurrent issues with comment as diff reporting panics that are resistant to fixing due to the fact that the panic occurs in the template render and is swallowed by the template renderer.

This PR just adds some logging to force the panic to properly logged and re-propagates back up to the template renderer so we can actually detect what the issue is.

Signed-off-by: Andrew Thornton art27@cantab.net