Raise an exception when the style does not exist

[olekscode]

Example to reproduce

presenter := SpPresenter new.
presenter application: (app := SpApplication new).

presenter layout: (SpBoxLayout newTopToBottom
	add: (label := presenter newLabel);
	yourself).
	
label label: 'Hello Moose people!'.
label addStyle: 'nonExistingStyle'.

presenter openWithSpec.

This should raise an error that a nonExistingStyle is missing