These commits do 3 things:

• Replace all occurrences of Control.Monad.Except with one or more of Control.Monad, Control.Monad.Error.Class (MonadError(..)), and Control.Monad.Trans.Except, since Control.Monad.Except seems to just re-export all of these. This is necessary in the case where we are using transformers==0.3.*, because transformers-compat doesn't have Control.Monad.Except (not sure why). I think this is worth doing anyway; I don't like how Control.Monad.Except re-exports so much stuff.
• Add an explicit dependency on transformers-compat. purescript was previously bringing it in as a transitive dependency, so the only effect that this should have is allowing the importing of its modules where necessary.
• Relax transformers lower bound to 0.3.*; now that we have transformers-compat, purescript can compile with transformers==0.3.*. I tested this by doing:

cabal sandbox delete
cabal sandbox init
cabal install --only-dependencies --enable-tests --constraint='transformers ==0.3.*'
cabal build
cabal test

The effect of this is that you can now easily install yesod-bin together with purescript in a sandbox. Also, it is still possible to get transformers==0.4.* when resolving constraints - these changes appear to have no effect on the versions selected in those cases which currently can be resolved.