How do I submit bug reports in Atom feeds?

We are implementing a REST interface and we decided to use application / atom + xml as the output format, as it gives us a standard, standard way to return a list of items along with links to related parts.

However, we are struggling to find a standard way to return errors. I know setting the correct HTTP error code will be required, but what about the message? The Atom 1.0 RFC does not mention error handling at all, The Atom RFC Publishing Protocol says that the response should contain a human-readable object with an error message. However, this does not help much. What MIME type should be the ideal answer? Plain text? HTML?

Is it good that if the client sends Accept: application / atom + xml, we return Content-Type: something else? Or would you prefer to include this message in your Atom entry? I would like to emphasize that the interface is for machines and not for users.

Thanks a lot for your suggestions.