Update lighter/system.py

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

lighter/system.py

@@ -207,7 +207,7 @@ def _base_step(self, batch: Dict, batch_idx: int, mode: str) -> Union[Dict[str,
 
         # If the loss is a dict, the sublosses must be combined under "total" key.
         if isinstance(loss, dict) and "total" not in loss:
-            raise ValueError("The loss dictionary must have 'total' loss, combining all the sublosses.")
+            raise ValueError("The loss dictionary must include a 'total' key that combines all sublosses. Example: {'total': combined_loss, 'subloss1': loss1, ...}")
 
         # Logging
         self._log_stats(loss, metrics, mode, batch_idx)