URL preview: warn on auth-related errors

[sknebel]

From some chat discussion right now about accidential public responses to private posts: https://chat.indieweb.org/dev/2022-02-28#t1646077696840800

Right now if a URL returns 401 Quill just doesn't show a preview for it, with no further indication that anything about trying to fetch it went "wrong". I'd be nice to get a clear message if there was an explicitly auth related error happening that warns the user that they might not want to post a reply publicly.

/cc @fluffy-critter