Stop a cancelled backup at once under --skip-errors (closes #286)

After Ctrl-C or SIGTERM, phase 2 of a --skip-errors backup treated the
cancellation error from each remaining file like an unreadable file: it
opened the file, printed an error line, counted it as failed and moved
on to the next. The processing loop now checks for cancellation before
each file, and an error from a file once the run is cancelled stops the
run instead of being skipped. The loop check is what stops a run
cancelled while an empty file is open, since an empty file has no
chunks and nothing reads the context while it is backed up.

The --skip-errors test helper now takes the scan's context.

Model: opus-5-5

TODO.md

@@ -22,6 +22,14 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-08: Made a backup cancelled under `--skip-errors` stop at once
+  ([issue #286](https://git.eeqj.de/sneak/vaultik/issues/286)). After
+  Ctrl-C or SIGTERM, phase 2 treated the cancellation error from each
+  remaining file like an unreadable file: it opened the file, printed an
+  error line for it, counted it as failed and went on to the next one.
+  The run now stops at the first file after the cancellation, and no
+  file is counted as failed because of it.
+
 - 2026-10-08: Made `remote nuke` delete the `.partial` files that
   uploads cut off part-way leave on the destination store
   ([issue #281](https://git.eeqj.de/sneak/vaultik/issues/281)). The