Bugfix to failReadE in Distribution.ReadE

Currently, `failReadE "some error"` returns `ReadE Left` --- dropping the supplied error message on the floor. This changes it to return ` ReadE (const ( Left "some error"))`, which I assume is what's intended.

Not the biggest deal in the world, as so far as I can tell failReadE isn't actually used anywhere. But it might save some confusion down the road.