Comment 5 for bug 37729

Well, it is not very clear a message has been raised, because the upper section of the screen remains the same and a user might confuse it like "oh seen that already, what has happened?".
A popup dialog for warning and errors is always better in my opinion, except when they are hindering the users interaction, like a 404 error in a webbrowser.

I will have a look one of these days what the Gnome HIG has to say about this.