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.
1 job for ea/fix-monad-fail in 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #31357
debian-stable-gcc

00:00:47