Provide a more instructive warning when ghc-mod is not installed

When I first installed your Atom package, I didn't quite read the instructions and got an obscure error message. Eventually I figured out that I didn't have ghc-mod installed.

I added check to see if the command failed because the executable was not found. I haven't worked with coffeescript before, so if you have suggestions on how to this differently, I'm happy to update the code and resubmit.