Add snapshot verify and restore to the commands a missing marker stops

Given a remote key, `snapshot verify` and `snapshot restore` resolve it
through `listAllRemoteSnapshotKeys`, which checks the destination marker,
so they fail when the marker is missing. Given a snapshot ID they do not
check it. Add them to the "These commands fail" list in `README.md` and
`docs/REPOSTRUCTURE.md`.

Model: fable-5-1

docs/REPOSTRUCTURE.md

@@ -59,6 +59,7 @@ not exist, commands treat the destination as not present:
     - `prune`
     - `snapshot purge`
     - `remote nuke`
+    - `snapshot verify` and `snapshot restore`, when given a remote key
 - `snapshot list` shows the local index alone.
 - `snapshot remove` deletes nothing at the destination.