editor: display detailed error messages in the infobar

The infobar used to display a static message regardless of the actual error. Let's use the underlying system error as infobar's subtitle.

Fixes #577 (closed).

I'm tempted to add a "Open" button, similarly to what is done with the "suggest admin" thing. But I doubt the benefits are worth the overhead, let me know what do you think.

I'm not really used to neither Gtk nor the coding style, but I'll gladly make changes if needed.