Given MonadError[F, Z], EitherT[F, E, ?] should provide a MonadError[..., Either[Z, E]]

[oleg-py]

The title says it all, basically.

It's a known fact that something like EitherT[IO, E, ?] has two channels of failing: by raising the Throwable in IO and by raising a E in an inner Either. Instead of prioritizing the former, I suggest we expose the ability to work with both channels of failure directly by using a coproduct type.

Same applies to OptionT[F, A]

[barambani]

I can give a look if that's ok, this is still in discussion may be ? Ignore then.