failover: tolerate rsync's benign vanished-file exit code in backup syncs

Both sync-sd-backup.sh and sync-usb-backup.sh used set -e, so rsync
returning exit 24 (partial transfer due to vanished source files --
expected on a live root, e.g. a container's shm socket disappearing
mid-sync) aborted the script before it printed its own success message,
even though the actual file transfer completed fully. Only exit 24 is
now tolerated; any other rsync failure still aborts as before.

Found running sync-sd-backup.sh today: it transferred all 919k files
correctly but exited non-zero and skipped its completion line.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YJfEELeh3ercpRBp8yYrYS

failover/sync-sd-backup.sh

@@ -38,12 +38,24 @@ cleanup() {
 }
 trap cleanup EXIT
 
+# rsync exit 24 ("partial transfer due to vanished source files") is expected on a
+# live root — files like container shm sockets or in-progress logs routinely disappear
+# mid-sync. Tolerate that one code; anything else is a real failure and still aborts.
+rc=0
 rsync -aHAX --numeric-ids --delete \
     --info=progress2 \
     --exclude=/proc/* --exclude=/sys/* --exclude=/dev/* --exclude=/run/* \
     --exclude=/tmp/* --exclude=/mnt/* --exclude=/media/* --exclude=/lost+found \
     --exclude=/boot/firmware/* --exclude=/swapfile \
     --exclude=/etc/fstab --exclude=/etc/machine-id --exclude=/var/log/* \
-    --stats / "$MOUNT_POINT/"
+    --stats / "$MOUNT_POINT/" || rc=$?
+
+if [ "$rc" -ne 0 ] && [ "$rc" -ne 24 ]; then
+    echo "rsync failed (exit $rc)." >&2
+    exit "$rc"
+fi
+if [ "$rc" -eq 24 ]; then
+    echo "Note: some files vanished mid-sync (rsync exit 24) — normal on a live root." >&2
+fi
 
 echo "SD backup ($SD_ROOT_PART) synced from $root_src."