[nodejs] cannot handle error when watchman is not installed

[From the docs:](https://facebook.github.io/watchman/docs/nodejs.html#checking-for-watchman-availability)

```js
var watchman = require('fb-watchman');
var client = new watchman.Client();
client.capabilityCheck({optional:[], required:['relative_root']},
function (error, resp) {
if (error) {
// error will be an Error object if the watchman service is not
// installed, or if any of the names listed in the `required`
// array are not supported by the server
console.log('watchman not installed');
console.error(error);
}
// resp will be an extended version response:
// {'version': '3.8.0', 'capabilities': {'relative_root': true}}
console.log(resp);
});
```

If watchman is not installed, what actually happens is an `Unhandled 'error' event` is thrown and cannot be caught. The only way to catch this is to add a `client.on('error', ...)` callback. This is doable but then further precaution needs to be taken to avoid catching other errors in that callback. It would be much more convenient if it worked as documented. I have a minimal reproduction example here: https://github.com/robrichard/watchman-error/blob/master/index.js

Start with the minimal reproduction in index.js and compare its client.capabilityCheck behavior with the linked Watchman Node.js documentation. Trace how the fb-watchman client reports a missing Watchman service, then verify that the documented callback receives the error without an unhandled 'error' event.

Written by the indexing model from the issue text.