Bug 991573 - Copy improvements in Call Barring service through MMI

```js
message.result = this._(mmiResult.statusMessage);
// If we are just querying the status of the service, we show a
// different message, so the user knows she hasn't change anything
if (sentMMI === CALL_BARRING_STATUS_MMI_CODE ||
    sentMMI === CALL_WAITING_STATUS_MMI_CODE) {
  if (mmiResult.statusMessage === 'smServiceEnabled') {
    message.result = this._('ServiceIsEnabled');
  } 
  // ...
```
This way, we don't need the same ```else``` statement twice.