Consider returning CantTell on unanswered Applicability questions

[Jym77]

When an Applicability question is not answered, Alfa returns Inapplicable.
We might instead be in a better place returning CantTell, as it conveys the information that some question was not answered. It seems to better align to the semantics of the outcomes, what other tools are doing, and how ACT rules evaluate consistency of implementations.