Put quotes around program names in some error messages

One user reported being rather confused by the message:

  cabal: The program happy version >=1.18.5 is required but it could
  not be found.

I guess they must have been wondering what a happy version was. They
said that this is much clearer:

  cabal: The program 'happy' version >=1.18.5 is required but it could
  not be found.

So that's what we now do.