Web login: detect when a fediverse instance is blocking us and show a better error message

[squigjess]

When connecting my Mastodon account in brid.gy then togging the Bridging switch on, a Bad Gateway page pops up, then shows the following error:

403 Client Error: Forbidden for url: https://cathode.church/users/squig ; {"error":"Public key not found for key https://fed.brid.gy/fed.brid.gy#key"}

I have one other account linked—a Bluesky account—but this shouldn't make too much of a difference, no?

[snarfed]

Hey, sorry for the trouble here! Looks like your instance is blocking Bridgy Fed. On https://cathode.church/about , if you expand the Moderated servers, it shows brid.gy as Suspended.

Ideally we'd show a better error message when this happens! It's not easy to detect when instances have blocked us, but I can try.

[snarfed]

Btw, it looks like their rationale for blocking us was based on a (common) misunderstanding. We've always been opt in. We floated the idea of opt out before launch, but never actually did it. Feel free to tell your admins that and ask them to unblock us if you want!