Logging: log `tryfile` when redirecting to index file on 404 handler
This will help us to know what projects are using an old feature where we served
`README.html` as it was an `index.html` page.
Eventually, it would be good to remove this "feature" (based on usage we can
grab from these logs) because it doubles the requests/checks we have to do on
our S3 bucket, increasing the total response time but also the costs.