1090867 - [Messages][Refactoring] Don't consider cancelled activity as u...

...nhandled error r=azasypkin
I still think it would be better to unify it a bit, smth like this:

* In ```requestAttachment```:
  * instead of this ```requestProxy.onerror('file too large');``` use ```requestProxy.onerror(new Error('file too large'));```;
  * instead of ```requestProxy.onerror.apply(requestProxy, arguments);``` use ```requestProxy.onerror.call(requestProxy, activity.error);```

* In ```_onAttachmentRequestError``` we then can use something like this (with this we can also cover ```pick cancelled``` DOMError that is fired when app cancels activity by itself):
```
var errorId = e instanceof DOMError ? e.name : e.message;
```

What do you think?

Btw, we definitely need to migrate ```requestAttachment``` to use ```Promises```, but it's up to you to do it or not :)