Unhelpful error if `mfpx set` is not run.

[andrewferrier]

If mfpx set has not been run, this is the error shown:

events.js:85
      throw er; // Unhandled 'error' event
            ^
Error: spawn /Applications/IBM/MobileFirst-CLI-7.2-dev/mfp ENOENT
    at exports._errnoException (util.js:746:11)
    at Process.ChildProcess._handle.onexit (child_process.js:1053:32)
    at child_process.js:1144:20
    at process._tickCallback (node.js:355:11)
    at Function.Module.runMain (module.js:503:11)
    at startup (node.js:129:16)
    at node.js:814:3

Would be helpful to show something more descriptive.