Do not mark URLs a fail when error is due to missing network connection
Don't dispatch cancelAll calls. 

- should fix pre-fetcher problems #247
- cancelAll should be always called using the main thread
Merge pull request #268 from matej/prefetcher-fix

Don't dispatch cancelAll calls. 
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  Don't dispatch cancelAll calls.
  Do not mark URLs a fail when error is due to missing network connection