Avoid premature completion of prefetcher if request fails

This addresses a number of issues, namely:

- This resolves bug in prefetcher that could result in premature notification of completion if one of the requests failed, as discussed in issue https://github.com/rs/SDWebImage/issues/715.

- This attempts to make documentation in headers a little less ambiguous regarding the intent of the `finishedCount` parameter, making it clear that it includes both successful and unsuccessful requests.

- Unrelated, also fix race condition in `SDWebimageDownloader`.