An agent (notably Last.fm's album.getInfo) can advertise a cover URL that is
itself dead — a 404. sources.go's fromURL returned a generic error for any
non-200, so a dead URL was treated as a transient failure: it churned in
backoff and counted toward the circuit breaker, stalling valid lookups.
Map 404/410 to model.ErrNotFound in fromURL so a dead URL settles absent, and
collapse the near-identical fetchPlaylistImageURL (which already did this for
M3U covers) into it.
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