Merge either into transformers

[Ross Paterson]

On Tue, Feb 25, 2014 at 12:33:27PM +0200, Roman Cheplyaka wrote:
> I proposed to merge either into transformers more than a year ago[1],
> and everyone seemed to agree. Could you please do it?
> 
> [1]: http://www.haskell.org/pipermail/libraries/2012-December/019027.html

Not everyone agreed when we discussed this last August.  My proposal then
was to introduce a new constructor ExceptT on the same pattern as ReaderT,
etc, and to deprecate ErrorT, and that's what I intend to include in the
next major release, after GHC 7.8 comes out.