fix: Handle non-retryable errors by returning None in simple_send_with_retries

aider/sendchat.py

@@ -88,10 +88,10 @@ def simple_send_with_retries(model_name, messages, extra_params=None):
                     should_retry = False
 
             if not should_retry:
-                break
+                return None
 
             print(f"Retrying in {retry_delay:.1f} seconds...")
             time.sleep(retry_delay)
             continue
         except AttributeError:
-            return
+            return None