When this code was written ExceptT didn't exist yet, but there's no reason to
vendor a custom implementation of it any longer. We're taking very little
advantage of the monads-tf features so just remove that dependency (and the
language extension reliance that goes with it).
This function was deprecated at some point in the distant past, and has
now been removed from gnutls upstream. There is no good replacement, and
no current users of the Haskell wrapper, so the easiest solution is to
remove setPriority from the binding.
