Hoist between ExceptT error types

[lrworth]

Enables going ExceptT e m a -> ExceptT e' m a.

[JackKelly-Bellroy]

Overlaps are checked when an instance is demanded; do you have sample code showing that this works? It might cause an overlapping instance error, like the instance for PluckError e (Except e) m used to.

[lrworth]

Overlaps are checked when an instance is demanded; do you have sample code showing that this works? It might cause an overlapping instance error, like the instance for PluckError e (Except e) m used to.

I have successfully used this instance. Perhaps we should add a test file to this package that uses all the instances.

[endgame]

Suggested change

	instance Monad m => PluckError e (ExceptT e m) m where
	instance (m ~ n, Monad m) => PluckError e (ExceptT e m) n where

I think this is what we want