yt-dlp collapses an ffmpeg postprocessing failure to a bare
"Postprocessing: Conversion failed!" with no indication of which step
died, and without --verbose the underlying ffmpeg reason is never
emitted at all. describeDownloadError() appends the last non-noise
trailing lines (e.g. the [ModifyChapters] SponsorBlock re-encode) so
the error log and completion notification point at the actual cause
instead of a dead end.
debont80
5de4b40c1d