[downloader/dash] Add fragment retry capability

[downloader/dash] Add fragment retry capability

YouTube may often return 404 HTTP error for a fragment causing the
whole download to fail. However if the same fragment is immediately
retried with the same request data this usually succeeds (1-2 attemps
is usually enough) thus allowing to download the whole file successfully.
So, we will retry all fragments that fail with 404 HTTP error for now.