Video is marked failed and downloaded again every run when a subtitle comes back empty #958

What happens

If YouTube sends back an empty file for one subtitle track, Youtarr marks the whole video as failed, even though the video downloaded fine. It then removes the video from complete.list, so the next run downloads it again and fails the same way. This keeps happening until the video is ignored. The notification also lists the video as both downloaded and failed.

This was reported on Discord with video 9biGuhJUbOQ (yt-dlp 2026.08.19). That video has an uploaded en-US subtitle and an auto-generated en one. en-US downloaded fine, but the auto en came back empty:

[info] Writing video subtitles to: .../[9biGuhJUbOQ].en.vtt
ERROR: Did not get any data blocks
Recorded video failure {"youtubeId":"9biGuhJUbOQ"}
[info] Downloading video thumbnail 46 ...
WARNING: Skipping embedding en subtitle because the file is missing

Cause

Youtarr only knows a subtitle error isn't a video failure when yt-dlp follows it with WARNING: Unable to download video subtitles for ... (YtdlpErrorTracker.handleWarningLine, added in #793). yt-dlp only prints that warning when the subtitle download throws an error. An empty response doesn't throw: yt-dlp prints the ERROR, skips that subtitle, and keeps downloading the video. The code is the same in nightly 2026.10.10.233141, so updating yt-dlp won't fix it.

Suggested fix

Have YtdlpErrorTracker treat an ERROR that shows up after [info] Writing video subtitles to: (and before the next step that isn't a subtitle) as a subtitle error. If the video file exists, count the video as downloaded (and add to subtitleFailureCount) instead of failed.

Workaround

Ignore the video.

Refs: #793, #914