Exit non-zero when snapshot create --skip-errors skips anything it cannot read (closes #310)

A backup under --skip-errors printed its skip errors, then "Finished
successfully.", and exited 0, so a cron wrapper could not tell it from
a complete backup.

Each skipped entry, including a directory the walk could not list, is
now counted as examined and failed. If --skip-errors skipped anything
it could not read or list, the run still keeps its snapshots and
prunes as before, then fails with "Backup completed, but N file(s)
could not be read". A file removed during the run, even before the
walk reaches it, counts once as failed and leaves the exit status
alone; without the flag it no longer stops the backup. Excluded
entries are never counted.

Model: opus-5-5

internal/vaultik/snapshot_deleted_count_test.go

@@ -178,11 +178,13 @@ func TestBackupDoesNotCountFileInUnopenableDirectoryAsDeleted(t *testing.T) {
 	}
 
 	env.out.Reset()
-	require.NoError(t, env.v.CreateSnapshot(&vaultik.SnapshotCreateOptions{
+	err := env.v.CreateSnapshot(&vaultik.SnapshotCreateOptions{
 		SkipErrors: true,
 		Snapshots:  []string{"work"},
-	}))
-	assert.Contains(t, env.out.String(), filesLine(1, 0, 1))
+	})
+	require.ErrorIs(t, err, vaultik.ErrFilesUnreadable)
+	assert.Contains(t, env.out.String(),
+		"Files: 2 examined, 0 backed up, 1 unchanged, 1 failed.")
 }
 
 // A backup counts only files that the previous snapshot of its own name