Following up a comment by Matthew Paul Thomas on the Design: Dialogs, long operations, and error messages post, I'm experimenting with ways to show the error message in dialogs directly, without the need of hover or keyboard focus.
The hard part is inserting an error message inside the dialog layout in a way that's not aesthetically horrible. In particular, catering to the alignment issues of both short and long messages is complicated.
For now, I'm thinking about placing the message on the top.
This looks intrusive, as it moves the entire content downwards, but keep in mind that the dialog becomes insensitive during a long operation that can fail. So the user will not be trying to click anything in the moment the error appears.
What do you think? Suggestions?