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

...nhandled error r=azasypkin
typo: ```normally```

Also I know it's bikeshedding :), but we usually use ```//``` comment notation for such inline comments, aligned to the previous line and empty line above to separate from previous line of code, so it should be:
```
alert(navigator.mozL10n.get('files-too-large', { n: 1 }));

// 'pick cancelled' is the error thrown when the pick activity app is
// canceled normally.
```