Success on a non-existent URL in the submission preview?

[PABourdin]

Using this web page:
https://whedon.theoj.org/
to make a preview of our paper, I find that I get a nice green OK message:
"Paper compilation complete!"
even if I give a non-existing URL to our repository:
https://github.com/pencil-code/nonexisting
Of course, if I click the preview link, the page says "404 Not found"...

It would be good to indicate the user that the paper was actually NOT found!