decide on and document how errors should be handled in core code and in GUIs

[alessandrofelder]

This would probably take the form of something like

• core code raises exceptions where needed
• set up an abstract qt widgets for all BG widgets, listening to sys.excepthook and emitting a signal on exception
• napari (and other frontends) are set up to listen to that signal and display info/warning/error via napari accordingly