Cleanly report missing system GHC for standalone projects

[alanz]

See
haskell/vscode-haskell#110

Or should it somehow try to use a stack one? or nix, or hvr-repo.

[Anrock]

Hm, i think there is no sane default for this, so it should be configurable via setting like standalone-file-ghc-path or just reported as missing (can't use hie with standalone files then).

[alanz]

The vscode issue relates to hie-wrapper trying to run the ghc exe without checking first for its existence.