Relax the Either monad

[Yitzchak Gale]

An instance of Monad (Either a) is defined in
Control.Monad.Error. Unfortunately, that instance requires
the type "a" to be an instance of Error.

In fact, using Either for exception handling is only a
special case. The Either monad is more generally useful for
any complex calculation that needs to exit in the middle and
return a value, including multi-level exit.

(And no thank you, I would not like to obfuscate my
code by using Cont and callCC for that. The Either monad
is simple and clean.)

It is annoying to work around this problem, since one
often has to import Control.Monad.Error.
Could this restriction be removed?

The Error instance is not used, except to define fail.
I don't think it is really important to have a Monad
instance where fail references the Error instance;
throwError is much more clear.

But if others disagree, perhaps a synonym could be used
for Either in the context of exceptions. Anyway the
namespace there is a bit of a mess; one expects Error
to be the non-transforming version of ErrorT, but it is
not. How about Throws or Thrown, so I can say:

myFun :: a -> b -> c ->
   Error1 `Thrown` Error2 `Thrown` Error3 `Thrown`
   ReturnType

-Yitz