mtl->transformers replacement, supporting transformers 0.3 (ErrorT)

I had a patch to replace ErrorT by ExceptT, but another one was provided to ghc-events just last week.

Therefore, I have separated one patch which provides code to replace mtl by transformers (which implements ExceptT), and uses supports both ErrorT and ExceptT (Cabal-generated version information and #ifdef).
/ Jost