There should be a better error message when a part's adaptive marking refers to a part that doesn't exist

[christianp]

A colleague made a gap-fill, where gap 1 used gap 0 in adaptive marking, then deleted gap 0. On running the question, there's an error message along the lines of 'errorCarriedBackReferences of undefined'. It should say something more sensible, like 'gap 1 refers to gap 0, which doesn't exist'.