serveHash routed every non-cancel cache error into dangling(), which returns
ErrUnavailable and is then rendered as a placeholder at 200 OK. A cache-layer
fault was therefore indistinguishable from an album genuinely having no artwork,
and left no trace: a broken resize cache silently served placeholders for a
quarter of the library while the logs stayed clean.
Log the error before falling back, so the cause is recoverable from the logs.
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