Remove the wrong assertion:
  all (`Set.notMember` failedSet)  (tail newlyFailedIds)

This is wrong for much the same reasons as the part of the invariant
(in the previous patch) was wrong.

Suppose Q depends on P1 and P2 and the processing set intially is
{P1, P2}. Now suppose that P1 fails, so the failure set becomes {P1,Q}
and the new processing set is {P2}. Now suppose P2 also fails. Then the
newlyFailedIds is [P2, Q] and so the tail is [Q]. Of couse Q is in the
failure set already, in violation of the above assertion. But this is a
perfectly reasonable scenario. It's just the assertion that is wrong.