[arte.tv:+7] Not available now : bad error message

[Shnoulle]

AttributeError: 'list' object has no attribute 'items' if not available currently (but in future).
Related to #13945

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• I've verified and I assure that I'm running youtube-dl 2017.09.02 (tested with last fedora version and local install version)
• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones ( this issue

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

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• Single video: https://www.arte.tv/fr/videos/075172-000-A/chromosome-3 (to do before 2017-09-07 )

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Description of your issue, suggested solution and other information

If a video are listed but not available currently : youtube-dl broke and didn't show proper error.

$ youtube-dl --verbose --dump-pages -F https://www.arte.tv/fr/videos/075172-000-A/chromosome-3
[debug] System config: ['--prefer-free-formats']
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--verbose', '--dump-pages', '-F', 'https://www.arte.tv/fr/videos/075172-000-A/chromosome-3']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.09.02
[debug] Python version 3.6.2 - Linux-4.12.9-300.fc26.x86_64-x86_64-with-fedora-26-Twenty_Six
[debug] exe versions: ffmpeg 3.3.3, ffprobe 3.3.3, rtmpdump 2.4
[debug] Proxy map: {}
[arte.tv:+7] chromosome-3: Downloading webpage
[arte.tv:+7] Dumping request to https://www.arte.tv/fr/videos/075172-000-A/chromosome-3
PCFkb2N0eXBlIGh0bW[…cut…]4K
[arte.tv:+7] chromosome-3: Downloading JSON metadata
[arte.tv:+7] Dumping request to https://api.arte.tv/api/player/v1/config/fr/075172-000-A?autostart=1&lifeCycle=1
ewogICAgInZpZG[…cut…]gICAgfQp9
Traceback (most recent call last):
  File "/usr/bin/youtube-dl", line 11, in <module>
    load_entry_point('youtube-dl==2017.9.2', 'console_scripts', 'youtube-dl')()
  File "/usr/lib/python3.6/site-packages/youtube_dl/__init__.py", line 464, in main
    _real_main(argv)
  File "/usr/lib/python3.6/site-packages/youtube_dl/__init__.py", line 454, in _real_main
    retcode = ydl.download(all_urls)
  File "/usr/lib/python3.6/site-packages/youtube_dl/YoutubeDL.py", line 1957, in download
    url, force_generic_extractor=self.params.get('force_generic_extractor', False))
  File "/usr/lib/python3.6/site-packages/youtube_dl/YoutubeDL.py", line 775, in extract_info
    ie_result = ie.extract(url)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/common.py", line 434, in extract
    ie_result = self._real_extract(url)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/arte.py", line 211, in _real_extract
    return self._extract_from_webpage(webpage, video_id, lang)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/arte.py", line 254, in _extract_from_webpage
    return self._extract_from_json_url(json_url, video_id, lang, title=title)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/arte.py", line 118, in _extract_from_json_url
    for format_id, format_dict in vsr.items():
AttributeError: 'list' object has no attribute 'items'

Same if i don't want to list format

$ youtube-dl --verbose https://www.arte.tv/fr/videos/075172-000-A/chromosome-3
[debug] System config: ['--prefer-free-formats']
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--verbose', 'https://www.arte.tv/fr/videos/075172-000-A/chromosome-3']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.09.02
[debug] Python version 3.6.2 - Linux-4.12.9-300.fc26.x86_64-x86_64-with-fedora-26-Twenty_Six
[debug] exe versions: ffmpeg 3.3.3, ffprobe 3.3.3, rtmpdump 2.4
[debug] Proxy map: {}
[arte.tv:+7] chromosome-3: Downloading webpage
[arte.tv:+7] chromosome-3: Downloading JSON metadata
Traceback (most recent call last):
  File "/usr/bin/youtube-dl", line 11, in <module>
    load_entry_point('youtube-dl==2017.9.2', 'console_scripts', 'youtube-dl')()
  File "/usr/lib/python3.6/site-packages/youtube_dl/__init__.py", line 464, in main
    _real_main(argv)
  File "/usr/lib/python3.6/site-packages/youtube_dl/__init__.py", line 454, in _real_main
    retcode = ydl.download(all_urls)
  File "/usr/lib/python3.6/site-packages/youtube_dl/YoutubeDL.py", line 1957, in download
    url, force_generic_extractor=self.params.get('force_generic_extractor', False))
  File "/usr/lib/python3.6/site-packages/youtube_dl/YoutubeDL.py", line 775, in extract_info
    ie_result = ie.extract(url)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/common.py", line 434, in extract
    ie_result = self._real_extract(url)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/arte.py", line 211, in _real_extract
    return self._extract_from_webpage(webpage, video_id, lang)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/arte.py", line 254, in _extract_from_webpage
    return self._extract_from_json_url(json_url, video_id, lang, title=title)
  File "/usr/lib/python3.6/site-packages/youtube_dl/extractor/arte.py", line 118, in _extract_from_json_url
    for format_id, format_dict in vsr.items():
AttributeError: 'list' object has no attribute 'items'

Didn't know date of availability can be shown, can be great but not necessary. Maybe just show : See on webpage if available