python: Catch JSONDecodeError instead of ValueError when decoding JSON.

These weren’t wrong since orjson.JSONDecodeError subclasses
json.JSONDecodeError which subclasses ValueError, but the more
specific ones express the intention more clearly.

(ujson raised ValueError directly, as did json in Python 2.)

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/outgoing_webhook.py

@@ -272,7 +272,7 @@ def process_success_response(event: Dict[str, Any],
                              response: Response) -> None:
     try:
         response_json = json.loads(response.text)
-    except ValueError:
+    except json.JSONDecodeError:
         fail_with_message(event, "Invalid JSON in response")
         return