Stop snapshot purge when a snapshot cannot leave the local index (closes #337)

snapshot purge, and the purge step of snapshot create --prune, logged a
failed local index delete, then deleted that snapshot's metadata from
the destination store anyway and exited 0, leaving a local record whose
metadata was gone. It now stops at that snapshot with an error, before
touching its metadata, so the command exits non-zero; snapshots already
purged stay purged. This matches how the same loop already handles a
failed metadata delete.

Model: opus-5-5

README.md

@@ -339,7 +339,9 @@ local index alone, and still exits zero.
 
 **`snapshot purge`**: Remove old snapshots based on criteria. Retention is
 per-snapshot-name (`--keep-latest` keeps the latest of each name, not the
-latest globally).
+latest globally). If a snapshot cannot be removed from the local index, the
+command stops there, leaves that snapshot's metadata on the destination store
+and exits non-zero; `snapshot create --prune` fails the same way.
 * `--keep-latest`: Keep only the most recent snapshot of each name
 * `--older-than <duration>`: Remove snapshots older than duration (e.g. `30d`,
   `4w`, `6mo`, `1y`; `m` is minutes, `mo` is months)