Bug 918970 - display existing thread when sending a sms from contact app

old PR : https://github.com/mozilla-b2g/gaia/pull/16216

nit: here is how you can write this instead:
```javascript
promise.then(() => {
  throw new Error('Should have rejected the promise');
}, (error) => {
  assert.ok(error);
}).then(done, done);
```
We also have https://github.com/domenic/chai-as-promised/ for few days, so maybe this works:
```javascript
var promise = MessageManager.findThreadFromNumber('123');
assert.isRejected(promise).notify(done);
```
I haven't tried yet so please try and tell me :)