Handle video posts with no thumbnail more gracefully

[fireattack]

Describe the bug
Currently if a post has no thumbnail, it returns 410 (it happens even on web, so 410 itself isn't an issue of instaloader) and the download fails.

To Reproduce
--login={username} -- -BkQQ-fKhZe9

(https://www.instagram.com/tv/BkQQ-fKhZe9/ or the oldest post of https://www.instagram.com/natgeo/channel/, you can see it indeed doesn't have a thumbnail.)

Expected behavior
The video itself should still be downloaded.

Error messages and tracebacks

Exception has occurred: ConnectionException
HTTP error code 410.
  File "G:\_temp\instaloader\instaloader\instaloadercontext.py", line 577, in get_raw
    raise ConnectionException("HTTP error code {}.".format(resp.status_code))
  File "G:\_temp\instaloader\instaloader\instaloadercontext.py", line 585, in get_and_write_raw
    self.write_raw(self.get_raw(url), filename)
  File "G:\_temp\instaloader\instaloader\instaloader.py", line 261, in download_pic
    self.context.get_and_write_raw(url, filename)
  File "G:\_temp\instaloader\instaloader\instaloader.py", line 79, in call
    return func(instaloader, *args, **kwargs)
  File "G:\_temp\instaloader\instaloader\instaloader.py", line 527, in download_post
    downloaded = self.download_pic(filename=filename, url=post.url, mtime=post.date_local)
  File "G:\_temp\instaloader\instaloader\__main__.py", line 153, in _main
    instaloader.download_post(Post.from_shortcode(instaloader.context, target[1:]), target)
  File "G:\_temp\instaloader\instaloader\__main__.py", line 423, in main
    _main(loader,
  File "G:\_temp\instaloader\instaloader.py", line 6, in <module>
    main()

Instaloader version
4.4.4

[aandergr]

Fixed in Version 4.5.1.