Added check for enoent error

This package is used by [testling](https://github.com/substack/testling) to launch a browser headlessly. When run on OSX, which doesn't have Xvfb installed by default, the call to spawn triggers an error event which is unhanded and displays this useless output:

```
events.js:72
        throw er; // Unhandled 'error' event
              ^
Error: spawn ENOENT
    at errnoException (child_process.js:980:11)
    at Process.ChildProcess._handle.onexit (child_process.js:771:34)
```

Added an listener for "error" that will pass back a useful error message when it can.