I would do this properly or not at all. By properly I mean to set css class and add icon using css, and also probably a title attribute.

You could try return html::div('boxwarning', $msg);.

I think this will introduce a dependency on a new syncroton release because otherwise the "broken" column will not be available.

I think it needs to be done, but I don't know how to do the styling, and I didn't find an example to follow along, so this is now a TODO.