[hlsnative] Add an ability to skip unavailable fragments

[dstftw]

• Bug report (encountered problems with youtube-dl)

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PS C:\dev\youtube-dl\master> youtube-dl.exe https://www.twitch.tv/naysayer88/v/85713845 -
v
[debug] System config: []
[debug] User config: []
[debug] Command-line args: ['https://www.twitch.tv/naysayer88/v/85713845', '-v']
[debug] Encodings: locale cp1251, fs mbcs, out cp866, pref cp1251
[debug] youtube-dl version 2016.08.24.1
[debug] Python version 3.4.4 - Windows-10-10.0.10240
[debug] exe versions: ffmpeg N-81192-g04da20e, ffprobe N-81192-g04da20e, rtmpdump 2.4
[debug] Proxy map: {}
[twitch:vod] 85713845: Downloading vod info JSON
[twitch:vod] 85713845: Downloading vod access token
[twitch:vod] 85713845: Downloading m3u8 information
[debug] Invoking downloader on 'http://vod.edgecast.hls.ttvnw.net/v1/AUTH_system/vods_60f4/naysayer88_23003365072_506378255/chunked/index-dvr.m3u8'
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 43
[download] Destination: Gammin' - Obduction Spoilerama-v85713845.mp4
[download]  34.9% of ~13.53MiB at Unknown speed ETA Unknown ETAERROR: unable to download video data: HTTP Error 400: Bad Request
Traceback (most recent call last):
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmpvarrvgfb\build\youtube_dl\YoutubeDL.py", line 1694, in process_info
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmpvarrvgfb\build\youtube_dl\YoutubeDL.py", line 1636, in dl
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmpvarrvgfb\build\youtube_dl\downloader\common.py", line 354, in download
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmpvarrvgfb\build\youtube_dl\downloader\hls.py", line 102, in real_download
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmpvarrvgfb\build\youtube_dl\downloader\common.py", line 354, in download
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmpvarrvgfb\build\youtube_dl\downloader\http.py", line 58, in real_download
  File "C:\Users\dst\AppData\Roaming\Build archive\youtube-dl\rg3\tmpvarrvgfb\build\youtube_dl\YoutubeDL.py", line 1996, in urlopen
  File "C:\Python\Python34\lib\urllib\request.py", line 470, in open
    response = meth(req, response)
  File "C:\Python\Python34\lib\urllib\request.py", line 580, in http_response
    'http', request, response, code, msg, hdrs)
  File "C:\Python\Python34\lib\urllib\request.py", line 508, in error
    return self._call_chain(*args)
  File "C:\Python\Python34\lib\urllib\request.py", line 442, in _call_chain
    result = func(*args)
  File "C:\Python\Python34\lib\urllib\request.py", line 588, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 400: Bad Request

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Twitch may serve unavailable fragments causing download to fail while using hlsnative.
ffmpeg skips unavailable fragments by default. Same feature should be available in youtube-dl.