vmmonad: added Monad.Fail compliance for Monad Mnd solving #5
With GHC 8.6, [the option -XMonadFailDesugaring is enabled by default](https://ghc.haskell.org/trac/ghc/wiki/Migration/8.6#MonadFailDesugaringbydefault), causing partial pattern matches in `do`-statement to desugar with the `fail` method from the `MonadFail` class. * src/VMMonad.hs: here.