If a feed gives a 404, should it say “no feed found” or report the actual error?

App
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Was trying to add this feed:

https://thereader.mitpress.mit.edu/feed/

Which works in-browser and shows the XML, but doesn’t work in NetNewsWire — instead you get the generic “Feed Not Found.”

I thought that was odd, but then trying different feed validators, some said it was fine:

https://rss.app/r/feed/Q64MtprGEr1N5CNo?utm_source=rssvalidator&utm_medium=website

But others, like the w3 org validator returned a 403 error:

My guess is this is what NetNewsWire got too — but the “No feed found” isn’t technically true? It’s just erroring? Feels like it should show if there’s a 403 or not? Vs. just a 404?