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
--skip-errors skipped anything it could not read or list, the run keeps
its snapshots, runs the post-backup prune as before, and returns an
error the command reports as "Backup completed, but N file(s) could not
be read". A file removed during the run, including one the walk finds
gone after listing its directory, is counted once, as failed, and
leaves the exit status at 0 with or without the flag; without it, such
a removal no longer stops the backup.
Model: opus-5-5
