Fix can't listener error event on incorrect file/directory

[leijuns]

Fix can't listener error event on incorrect file/directory.

Like this:

const watcher = watch('incorrect-some-file-path', function(evt, name) {
  //do something
});
watcher.on('error', () => {
  //do something
});

expose error:

events.js:353
      throw er; // Unhandled 'error' event

[yuanchuan]

Thank you for the patch! Would you fix the test?

[leijuns]

Fix test case, you can approval it.