return raw post rst when rendering fails

[geotre]

This PR introduces a simple change to show the raw RST text when a post fails to render (as discussed in #330). This should improve the experience when reading older threads where the issue is more prevalent, and improve visibility with search engines that can now pick up the original post content instead of the error message. It's been suggested that recent improvements to the RST parser will reduce the amount of these problematic posts as well.

I have used the content of the first post from this thread for testing.

Before:

After:

I've also tested that embedding the raw post does not allow for js injection.

[geotre]

CI fail is from a network error, unrelated to the changes