Fail snapshot remove when a destination-only snapshot's metadata cannot be deleted (closes #340)

When a snapshot was only on the destination store and deleting its
metadata there failed, snapshot remove warned and exited 0 having
removed nothing. That failure is now an error. A snapshot in the local
index keeps its behaviour: it is removed from the local index and the
failed delete on the destination store is a warning.

The README and the help text said the `vaultik prune` follow-up is
printed on every success; they now say it is printed only when metadata
was removed from the destination store, and never under --json. The
comments on RemoveSnapshot and removeSnapshotRemote describe both
cases; removeSnapshotRemote's also says it returns true only when the
delete finished without an error and removed at least one object.

Model: opus-5-5

internal/vaultik/snapshot.go

@@ -1399,11 +1399,11 @@ func (v *Vaultik) confirmRemoveSnapshot(snapshotID string, opts *RemoveOptions)
 // without an error and removed at least one object; under LocalOnly it
 // does nothing. When the snapshot is only on the destination store, a
 // failure is returned as an error, because that removal is all the
-// command does. When the snapshot was in the local
-// index, its removal there has already happened, so a failure is only a
-// warning telling the user to run `vaultik snapshot remove` for the
-// snapshot again once the destination store is reachable (`vaultik
-// prune` never removes snapshot metadata).
+// command does. When the snapshot was in the local index, its removal
+// there has already happened, so a failure is only a warning telling the
+// user to run `vaultik snapshot remove` for the snapshot again once the
+// destination store is reachable (`vaultik prune` never removes snapshot
+// metadata).
 func (v *Vaultik) removeSnapshotRemote(
 	snapshotID string, inLocalIndex bool, opts *RemoveOptions,
 ) (bool, error) {