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/cli/snapshot.go

@@ -270,8 +270,9 @@ By default, this removes the snapshot from the local index database and
 strips the snapshot's metadata from the backup destination store. Blobs
 are NOT touched: deleting them requires reading every remaining remote
 manifest (the destination store may hold snapshots this host doesn't
-know about), which is what 'vaultik prune' does. On success the command
-prints the exact 'vaultik prune' invocation to run as a follow-up.
+know about), which is what 'vaultik prune' does. The command prints the
+'vaultik prune' invocation to run as a follow-up only when it removed the
+snapshot's metadata from the destination store, and never under --json.
 
 An ID that names no snapshot in the local index or on the destination
 store is an error.
@@ -280,10 +281,12 @@ Use --local-only to skip the remote half (e.g. when you want to forget a
 snapshot locally without touching the destination store); the snapshot
 must then be in the local index.
 
-If the remote is unreachable, the local-database removal still completes
-and a warning is emitted; run 'vaultik snapshot remove <snapshot-id>' again
-once the destination store is reachable to remove the snapshot's metadata
-from it ('vaultik prune' does not).
+If removing the snapshot's metadata from the destination store fails (for
+example because the remote is unreachable), the local-database removal
+still completes and a warning is emitted; run 'vaultik snapshot remove
+<snapshot-id>' again once the destination store is reachable to remove
+the snapshot's metadata from it ('vaultik prune' does not). For a
+snapshot that is only on the destination store, that failure is an error.
 
 To wipe the entire destination store and start over, use 'vaultik remote
 nuke --force' — it is the single supported entry point for that.`,