Restore planning stopped at the first file needing a chunk that no blob
holds, so --skip-errors never saw that file and nothing was restored.
The planner now leaves such a file out of the plan and returns it with
the reason, and each one goes through handleRestoreFileError like any
other failed file: skipped and counted under --skip-errors, fatal
without it. The fatal error now suggests --skip-errors, as other restore
failures do.
The skip-errors tests now share one backup helper; the new case removes
a blob from the snapshot metadata and exports it again.
Model: opus-5-5
