Exit non-zero when snapshot create --skip-errors skips anything (closes #310)

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

README.md

@@ -178,7 +178,7 @@ vaultik version
 * `--verbose`, `-v`: Enable verbose output (on stderr — see below)
 * `--debug`: Enable debug output (on stderr — see below)
 * `--quiet`, `-q`: Suppress non-error output (also suppresses startup banner)
-* `--skip-errors`: Skip files that cannot be read when creating a snapshot, or that cannot be restored when restoring, instead of aborting. Packing and storage errors while creating a snapshot (which would leave a chunk recorded but not stored) still abort the run.
+* `--skip-errors`: Skip files that cannot be read when creating a snapshot, or that cannot be restored when restoring, instead of aborting. Packing and storage errors while creating a snapshot (which would leave a chunk recorded but not stored) still abort the run. A run that skipped anything still finishes but exits non-zero: `snapshot create` keeps the snapshot and counts each file or directory it skipped as failed.
 
 ### locking