We're not comfortable with the notion of error and warning messages being able to span multiple paragraphs, contain bulleted lists, tables, etc.  Suggest using the existing http://build.fhir.org/extension-rendering-markdown.html  extension if you want to provide a marked-up alternative.  We don't see support for complex markdown as falling within the scope of 'core' here. We will update the IGPublisher to support this extension and will talk to other tool builders (e.g. HAPI) to do the same.