Show critical error messages in user interface

[Oliver Sauder]

Currently critical error messages are only logged when Diodon is started on the console.

There are critical errors though which prevent items being added to Diodon and user doesn't have any idea what is happening when copying and not finding any items in the clipboard history.

Modal dialogs are not possible as this would cause user interruption when some hits Ctrl+C - an option would be though to show last critical error on the top of the clipboard history menu as a unselectable item.