Correctly ignore invalid / incomplete snapshots.

> Actually, no, the beOkResult does the right thing, it is just not nice. 

Oh right. I didn't realise it wasn't the normal `beOk`. Sigh.

> This is related to comment below vvv All these tests need a fair bit of attention. Can re-do them all now or leave for a second pass, thoughts?

Oh second pass is fine. They definitely need some love. Do we have an issue to track tests that need looking (or maybe add a FIX comment?)