* The note on how to prevent stable unfoldings from leading to
  `case` on `absentError` was wrong. Make it reflect reality.

* Reviewing the above change, Simon noticed that we didn't
  set an arity for `absentError`, which definitely has arity 1.
  It may not matter much, since `absentError` usually vanishes
  quickly, but we might as well set it properly, so now we do.

Reviewers: simonpj, bgamari

Reviewed By: simonpj

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D4655