GetArtistImages scrapes the og:image tag off the Last.fm artist page, because
the API only ever returns the placeholder image. Last.fm now answers non-browser
clients with a Fastly bot challenge, served as a 200 with valid HTML, so the
query found no og:image and the agent returned an empty list with no error. The
artwork worker read that as a definitive "this artist has no image" and settled
the state as absent, silently and with nothing in the log.
A real artist page always carries an og:image, so its absence now returns an
error instead. The worker keeps the previous state, other agents still get their
turn, and its per-agent circuit breaker bounds the retries. The error is
deliberately not a RetryLaterError: that would park the whole Last.fm agent,
including the API-backed biography, similar-artists and top-songs calls, which
the page block does not affect.
The new fixture is the real 3038-byte challenge page.
Fixes#6192
285dc4f391