cirrusdoc: Handle unrenderable documents

Not all pages / revisions are renderable. In particular there is no reprensentation for a redirect page, redirects are a property of the target page and not directly represented in the search index. If we somehow request a redirect to be rendered the backend will respond that it is unrenderable and we must then fail the request and not write the redirect to the index.

For those most part this code should never run, earlier handling should transform redirects into targets and we render the target. This handles the edge case where for some reason that didn't happen.

Bug: T385571