Handle uncaught exceptions from spawning dig process

[sarus]

Currently the library does not handle exceptions thrown when attempting to spawn the dig process making them hard to handle in code using the library.

This adds a listener to catch those exceptions.

Easiest way to test is to simply uninstall dig like this:

yum remove bind-utils -y

Even if you have a catch on your dig usage like this:

  it('Query A for google.com and return raw result', (done) => {
    dig(['google.com', 'A'], { raw: true })
      .then((result) => {
        expect(result).to.be.a('string');
        done();
      })
      .catch((err) => {
        console.log('Error:', err);
      });
  });

You will still get an uncaughtException error. In this case you will get a spawn dig ENOENT uncaught exception. Adding the handler allows these errors to be caught and handled in the catch block.

Wasn't entirely sure what the best way to write a test for this would be as I think it would require stubbing out the spawn call to just throw an exception but I think the error event should be handled.