fix: raise exceptions properly in process_chat_payload

backend/open_webui/utils/middleware.py

@@ -644,7 +644,7 @@ async def process_chat_payload(request, form_data, metadata, user, model):
             request, form_data, model, extra_params
         )
     except Exception as e:
-        return Exception(f"Error: {e}")
+        raise Exception(f"Error: {e}")
 
     tool_ids = form_data.pop("tool_ids", None)
     files = form_data.pop("files", None)