Fix redirection for pages missing in other docs versions

[drammock]

related #844
related #1343

I think this should fix our redirection problems. The error was that we assumed that a 404 response would be an error, but as far as Javascript is concerned a 404 response is still a real response! Now we check for response.ok (meaning a response in the 200-299 range) and triage the URL accordingly. The try/catch is left in place as a fallback in case something actually did go wrong with the request/response (i.e., on local or PR builds when the request is blocked due to CORS restrictions).

cc @lagru would appreciate if you could test this branch and report back

[12rambau]

The CORS issue is working, I'll merge it to check the build in release. Note that I changed the PR comment to avoid closing the issues if it doesn't work.

[12rambau]

tested on this page: https://pydata-sphinx-theme.readthedocs.io/en/latest/examples/gallery.html
It does not exist in V0.9 and the redirection worked as expected. Congrats, I'll close the associated issues.

[lagru]

At least locally this seems to resolve the issue! When I select a version that doesn't have an equivalent page it resolves to the root of that target version. I'll keep an eye on this and report back should the issue resurface for us.

@drammock, thanks for tackling this! :)