Do not prune after a backup that skipped unreadable files (closes #382)

snapshot create --skip-errors --prune pruned even when files were
skipped, so by default it deleted the previous snapshot of that name,
the only one still holding a file that had become unreadable, along
with blobs only that snapshot referenced.

The prune is now skipped exactly when the run exits non-zero for
entries --skip-errors could not read or list: older snapshots are
kept, a warning says why, and the run still exits non-zero. A file
removed during the run does not count, so such a run still prunes.
The README's --prune text and the flag's help say so.

Model: opus-5-5

internal/vaultik/snapshot.go

@@ -153,7 +153,15 @@ func (v *Vaultik) CreateSnapshot(opts *SnapshotCreateOptions) error {
 			len(snapshotNames), v.UI.Duration(time.Since(overallStartTime)))
 	}
 
-	if opts.Prune {
+	switch {
+	case opts.Prune && filesUnreadable > 0:
+		// An older snapshot may be the only one that still holds a file
+		// this run could not read.
+		log.Warn("Skipping post-backup prune",
+			"files_unreadable", filesUnreadable)
+		v.UI.Warningf("Skipped the post-backup prune because %d file(s) "+
+			"could not be read; older snapshots are kept.", filesUnreadable)
+	case opts.Prune:
 		err = v.runPostBackupPrune(snapshotNames, opts.KeepNewerThan)
 		if err != nil {
 			return fmt.Errorf("%w: %w", ErrPostBackupPruneFailed, err)