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

...nhandled error r=azasypkin
yeah, this won't work because ```assert.equal``` checks for equality of instances, but here we have two different ```Error``` instances. I think the following two asserts instead would be enough:
```
 assert.instanceOf(err, Error);
 assert.equal(err.message, 'file too large');
```