When a concurrent caller awaits `this.complete` inside `execute()` and
`cancel()` is called, the promise rejects with `undefined` outside of any
try/catch, causing "Uncaught (in promise) undefined" console spam during
rapid timeline scrolling.
- Wrap the `await this.complete` path in try/catch, returning 'CANCELED'
- Guard the `finally` block to only null `cancelToken` if it still belongs
to this call, preventing a race condition with `cancel()` to `init()`
Change-Id: I65764dd664eb408433fc6e5fc2be4df56a6a6964
