Add server error handler. Fail if address is already in use.

If you have the server running and then run `bug-clinic verify client.js` it always returns a PASS. 

Currently it throws the error:
````
events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: listen EADDRINUSE
````

Then proceeds to indicate success (regardless of your solution):

````
# PASS

Your solution to SCAN passed!
````

I've handled this particular error.