Completed user REST APIs

Completed user REST APIs
I find this a bit hard to read, can we not just do:
```javascript
if (err && err.code !== 404) {
    log().error({'err': err, 'host': host}, 'Failed to check if an app with a provided host name already exists');
    return callback(err);
} else if (app) {
    return callback({'code': 500, 'msg': 'An app with the provided host name already exists'});
}
```