EitherErr Applicative for Functor-Applicative-Monad_Proposal

`EitherErr' is an instance of Monad but not Applicative - this will become an error in GHC 7.10, under the Applicative-Monad Proposal.

http://www.haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal