There was code to handle unparseable messages from things like third party browser extensions but it caught a JSON parsing error, logged an error and then continued, which would throw an unhandled exception trying to read the `ack` property of `data` which had been left as `undefined`.
Add a `return` to abort execution of the handler function and ignore the message instead. Also changes the error to warning, since it's not a fatal error. Add a comment to clarify the behaviour.
From the existing comment, it looks like this was the original intention of the code, but the `return` was simply missed.
