vmmonad: add backwards-compatible MonadFail compliance

The `fail` method on `Monad` has been removed in `ghc` 8.8+, I added a
conditional compilation statement according to what was indicated in the
following wiki article:
<https://gitlab.haskell.org/ghc/ghc/wikis/proposal/monad-fail#adapting-old-code>

* src/VMMonad.hs: Here.
1 job for bb/fix-fail-monad in 3 minutes and 9 seconds (queued for 1 minute and 34 seconds)
Status Job ID Name Coverage
  Test
passed #82572
debian-stable-gcc

00:03:09