small fix for better error handling if the summarizer ever fails

- fix #231

aider/history.py

@@ -87,6 +87,8 @@ class ChatSummary:
         ]
 
         summary = simple_send_with_retries(self.model.name, messages)
+        if summary is None:
+            raise ValueError(f"summarizer unexpectedly failed for {self.model.name}")
         summary = prompts.summary_prefix + summary
 
         return [dict(role="user", content=summary)]