Test for `Left 0`

[tonymorris]

Anything that uses the ExitcodeT constructor should test that Left 0 never occurs.

[techtangents]

data ExitcodeT f a =
  ExitcodeT (f (Either Int a))

Perhaps replace the Int with a NonZero type? That probably just pushes Int -> Nonzero validation elsewhere, but at least Left 0 would be impossible in ExitCodeT.