PreviewError subclasses carry a concise short label for UI, a full log
message, backend name, and metadata fields (OnlyOffice code/status/url,
backend stage, timeout seconds, cancel reason). Exceptions travel across
the worker pool wire pickled in the binary response payload (ok=False)
and are re-raised with their original type on the caller side. Combined
pipelines such as pdf+pyvips tag the failing stage.
