Improve error reporting when proxies mess with the page

[surgeio]

• I've verified and I assure that I'm running youtube-dl 2016.10.07

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Description of your issue, suggested solution and other information

When using proxies to get around geo-location, there is a risk that the proxy will return a page different from the page you expect. See, for example, issue #10873. Unfortunately the error message given by youtube-dl is actively misleading: the error message explicitly tells you to report it as a bug. It would be very helpful if youtube-dl was smarter about this situation. I don't know exactly what could be done, but I have some ideas:

• if there is a proxy set, suggest that the user check whether the proxy is returning the right page before reporting it as a bug;

• perhaps youtube-dl could recognise the most common examples (e.g. phpinfo page?) and report a better error: "use a different proxy, this one redirects to phpinfo";

• how smart can youtube-dl get? is it capable of recognising that (say) a BBC url doesn't give a BBC page? It doesn't need to be 100% accurate, just accurate enough to avoid misleading the user and suggesting the problem is a bug in youtube-dl. I know there is a big gap between the ability of a human to recognise a re-directed page and what an application like youtube-dl can do, but it might be possible to close the gap even a little bit.

• a really dumb test which might catch 90% of the cases:

  from urlparse import urlparse
  domain = urlparse(url).hostname
  page = download(url)
  if domain not in page:
  # probably not on the real page, probably redirected to something unexpected

[yan12125]

how smart can youtube-dl get

Unfortunately it's not smart at all, so only the first option is feasible. A better comment can be:

if you can watch this video in browsers, please report this issue on https://yt-dl.org/bug