[dropbox] no error detected, invalid HTML downloaded

[shaicoleman]

• I've verified and I assure that I'm running youtube-dl 2016.04.01
• Bug report (encountered problems with youtube-dl)

$ youtube-dl -v 'https://www.dropbox.com/s/z79rgbge0w0pfi4/missing.mp4?dl=1'

[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'-v', u'https://www.dropbox.com/s/z79rgbge0w0pfi4/missing.mp4?dl=1']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2016.04.01
[debug] Python version 2.7.6 - Linux-4.4.0-14-generic-x86_64-with-Ubuntu-14.04-trusty
[debug] exe versions: ffmpeg N-79139-gde1a0d4, ffprobe N-79139-gde1a0d4
[debug] Proxy map: {}
[debug] Invoking downloader on u'https://www.dropbox.com/s/z79rgbge0w0pfi4/missing.mp4?dl=1&dl=1'
[download] Destination: missing-z79rgbge0w0pfi4.mp4
[download] 100% of 155.43KiB in 00:00

Description of your issue, suggested solution and other information

In some cases, youtube-dl doesn't detect invalid content, and downloads the HTML error page instead. In this case there's an issue also on Dropbox's side, as they don't return the correct HTTP code (200 instead of 404), but youtube-dl should recognize that it's not a video and throw an error