Abort execution if there are ad hoc attributes in specgen xml

[opoudjis]

I've resolved ad hoc attributes in #40 , detecting SIF_RefObject and removing the remainder in Specgen XML through Venetian Blinds refactoring.

But in anticipation of when @4pins uses these stylesheets, I need to make OpenAPI outright break when one of these ad hoc attributes is seen (other than SIF_RefObject), so that the specgen data model maintainer can be alerted to get rid of them from the data model.

[4pins]

Just let me know what to look for and what to do about it. We need good consistent outputs!