Make errors from compile() nicer

[shuhei]

Making errors from compile() nicer. I couldn't add a unit test for this because the error is thrown asynchronously and hard to catch.

Also updating the version in package-lock.json.

Before & After

With the following test script:

const compiler = require('.');

var opts = {
  pathToElm: "i-dont-exist",
  verbose: true,
  cwd: '',
};

compiler.compile("Parent.elm", opts);

Before:

[~/work/elm/node-elm-compiler] (master *) € node test.js
Running i-dont-exist make Parent.elm
/Users/shuhei/work/elm/node-elm-compiler/index.js:103
      .on('error', function(err) { throw (err); });
                                   ^

Error: spawn i-dont-exist ENOENT
    at _errnoException (util.js:992:11)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:190:19)
    at onErrorNT (internal/child_process.js:372:16)
    at _combinedTickCallback (internal/process/next_tick.js:138:11)
    at process._tickCallback (internal/process/next_tick.js:180:9)
    at Function.Module.runMain (module.js:695:11)
    at startup (bootstrap_node.js:191:16)
    at bootstrap_node.js:612:3

After:

[~/work/elm/node-elm-compiler] (master *) € node test.js
Running i-dont-exist make Parent.elm

/Users/shuhei/work/elm/node-elm-compiler/index.js:103
      .on('error', function(err) { throw compilerErrorToString(err); });
                                   ^
Could not find Elm compiler "undefined". Is it installed?