Worker logs expected PreviewError failures as warnings; tracebacks are
reserved for unanticipated internal errors. pdf read errors report
backend "pdf" (vips not yet reached); the stage field is dropped as the
backend name now encodes the failing pipeline step. Backend label
"pyvips" renamed to "vips" everywhere.
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.
LeoVasanko