Generate warning on `archive` with read-write snapshot

Firstly, thanks to `btrbk` and everyone involved in saving my skin. I b0rked my FS and was able to restore everything beautifully.

In the process, I noticed that I had accidentally set one snapshot to be read-write.

`btrfs send` requires snapshots to be RO. The `archive` just stopped silently at the RW snapshot.

Could a warning be given?

The issue identifies the archive operation and the btrfs send step as the relevant entry points, but it names no files or tests. Start by locating the archive implementation and its handling of read-write snapshots, then reproduce the case with a read-write snapshot. Done means archive emits a warning instead of stopping silently at that snapshot.

Written by the indexing model from the issue text.