Return error message when the completion model is not specified for the Jupyternaut persona (#1408)

* Return error message when the completion model is not specified for the Jupyternaut persona

* use early return for missing model check

---------

Co-authored-by: Andrii Ieroshenko <[email protected]>

Author: srdas

packages/jupyter-ai/jupyter_ai/personas/jupyternaut/jupyternaut.py

@@ -27,6 +27,12 @@ def defaults(self):
         )
 
     async def process_message(self, message: Message) -> None:
+        if not self.config_manager.lm_provider:
+            self.send_message(
+                "No language model provider configured. Please set one in the Jupyter AI settings."
+            )
+            return
+
         provider_name = self.config_manager.lm_provider.name
         model_id = self.config_manager.lm_provider_params["model_id"]