save-file/gui-error in editor:basic<%>

This method is an alternative to save-file. Rather than showing errors via the original stdout, it opens a dialog with an error message showing the error.

• (send an-editor:basic save-file/gui-error filename format show-errors?) -> boolean
     filename = #f : (union string #f)
     format = 'same : (union 'guess 'standard 'text 'text-force-cr same copy)
     show-errors? = #t : boolean