Stop a backup on a symlink whose target cannot be read (closes #269)

When readlink failed, the scanner logged at debug level and left the
symlink out of the snapshot, and the run reported success even without
--skip-errors. The error now goes through the same handling as any
other entry the walk cannot read: the run aborts, or with --skip-errors
the symlink is skipped with the usual "Failed to access" error line.

A symlink removed between the walk's lstat and the readlink also
aborts the run, as an entry that vanishes during the walk already does.

Model: opus-5-5

TODO.md

@@ -22,6 +22,13 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-07: Made a symlink whose target cannot be read stop the backup
+  ([issue #269](https://git.eeqj.de/sneak/vaultik/issues/269)). It was
+  left out of the snapshot with only a debug log line, even without
+  `--skip-errors`. It now aborts the run, or with `--skip-errors` is
+  skipped with the `Failed to access` error line that any other entry
+  the scan cannot read gets.
+
 - 2026-10-07: Stopped a killed rclone upload from leaving a truncated
   object under its key
   ([issue #266](https://git.eeqj.de/sneak/vaultik/issues/266)). The