Before this patch, GHC/Err.lhs-boot exported divZeroError and overflowError,
as well as plain 'error'.  The latter has a wired-in defn in GHC (MkId.lhs),
but the former two do not.  As a result GHC doesn't see that overflowError
is a bottoming function at a crucial moment when compiling GHC.Real, and
that means that divMod wasn't getting the CPR property.

The fix is easy:
  - GHC/Err.lhs-boot should export only 'error'

  - GHC.Real, GHC.Int, and GHC.Word should import GHC.Err
    directly.  They can do this nowadays without creating
    a module loop, thanks to the new exception story