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

Or something like that 

```
var viewInfo = this._findThreadIdByNumber(number)
    .then(function(threadId) {
        return {number: number, threadId: threadId};
    }).catch(function() { 
       return this._findContactByNumber(number).then(function(contact) { 
          return {number: number, contact: contact}; 
    });
 this.toView.bind(this, viewInfo);
```
And I raise an error in findThread if I don't find it.
I don't really like the the "isInteger" thing. Maybe it is because I come from typesafe languages, but what if someday we decide that we use contact ID? Well, anyway I'd prefer not to mix var of different types if I  can avoid it. I find it clearer (but again, maybe I'm just not used to).
What do you think?