Add pointer to original file when plugin/module doc page build fails

[bcoca]

The page still gets created but with the error output, not the original docs, so it should at least point to the original source for the page (which should have somewhat readable docs) and/or have a hint about using ansible-doc to attempt to display same from the local install as a fallback.

[felixfontein]

This will be possible with #355, which allows to add the 'Edit on GitHub' back.