A backup under --skip-errors printed its skip errors and failed count,
then "Finished successfully.", and exited 0, so a cron wrapper could
not tell it from a complete backup. A directory the walk could not list
was not counted at all.
Each entry the walk skips is now counted as examined and failed. When
anything failed under --skip-errors, the run keeps its snapshots, runs
the post-backup prune as before, and then returns an error that the
command reports as "Backup completed, but N file(s) failed to back up".
Judgement call: without --skip-errors a file removed during the run is
still counted as failed and the exit status stays 0.
Model: opus-5-5
