fix start.rs bail! string quoting (nested double quotes broke the macro — 9770012 shipped broken); same guard

src/docker/start.rs

@@ -48,7 +48,7 @@ pub(crate) async fn start_cluster(
     // "stalls". Refuse instead.
     if nnodes > 1 && recipe.distributed_backend.is_none() {
         bail!(
-            "recipe '{}' runs on {} nodes but sets no distributed_backend — add `distributed_backend = "mp"` (or "ray") so the launcher emits the rendezvous flags; otherwise every node would launch an independent single-node engine and try to load the full checkpoint",
+            "recipe '{}' runs on {} nodes but sets no distributed_backend — add distributed_backend = \"mp\" (or \"ray\") so the launcher emits the rendezvous flags; otherwise every node launches an independent single-node engine and tries to load the full checkpoint",
             recipe_name,
             nnodes
         );