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

...nhandled error r=azasypkin
nit: if line is longer than 80 chars, we format it like this:
```
sinon.assert.calledWithExactly(
  console.warn,
  'Unhandled error: ',
  new Error(err)
);
```

Also I've just noticed funny thing - try to change ```sinon.assert.calledWith(console.warn, 'Unhandled error: ', new Error(err));``` with eg. ```sinon.assert.calledWith(console.warn, 'Unhandled error: ', new Error('totally-different-error'));``` and test will pass  :)

Looks like some sort of incorrect behaviour inside sinon library, so I'd rather use smth like this to be on the safe side:
```
sinon.assert.calledWithExactly(
   console.warn,
   'Unhandled error: ',
    sinon.match.same(err)
);
```