Better support for throwing exceptions

[edsko]

At the moment acid-state uses

newtype Update st a = Update { unUpdate :: State st a }
    deriving (Monad, Functor, MonadState st)

newtype Query st a  = Query { unQuery :: Reader st a }
    deriving (Monad, Functor, MonadReader st)

Neither of these monads is particularly suitable for throwing exceptions. The only possibility is to use fail, but neither the Reader monad nor the (lazy) State monad is strict enough so that in

foo = do
    fail "The query will continue perfectly fine after this fail"
    restOfQuery

does what one would expect (restOfQuery gets executed). Ideally, there would be explicit support for exceptions (ErrorT, perhaps), with the expected strictness properties.