[foreign] RFR: Fix UndefinedLayoutException exception message

[Maurizio Cimadamore]

> Then try with
>
> scope.allocate(LayoutType.ofStruct(BadStruct.class)).get().foo()
>
>
Ok, the above also doesn't work - but this should?

scope.allocate(LayoutType.ofStruct(BadStruct.class).pointer()).get().get().foo() 


This time we should be able to bypass Scope.allocateInternal...

The main issue about having a cross header ref is that the test is gonna 
be sensitive to whatever we do in that area - e.g. that failure is, in a 
way, temporary; it would be preferrable to have a test that really 
checked something Bad (TM).

Maurizio