minor logging improvement

src/olmo_core/train/checkpoint.py

@@ -193,14 +193,15 @@ def load(
         if get_rank(self.process_group) == 0:
             try:
                 metadata = get_checkpoint_metadata(model_and_optim_dir)
-            except FileNotFoundError:
+            except FileNotFoundError as exc:
                 # Try base directory, which could be the case if user is trying to load model weights
                 # (possibly with optimizer state), and not an actual train checkpoint.
                 if trainer_state is None:
                     metadata = get_checkpoint_metadata(dir)
                     model_and_optim_dir = dir
                 else:
-                    raise
+                    raise FileNotFoundError(f"Missing checkpointing metadata in '{dir}'") from exc
+
             if load_optimizer_state is None:
                 for key in metadata.state_dict_metadata.keys():
                     if key.startswith("optim."):