Repository: youtube-dl
Issue: Be able to download metadata from DRM protected video
Labels: request

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## Checklist

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- [x] I'm reporting a feature request
- [x] I've verified that I'm running youtube-dl version **2019.11.22**
- [x] I've searched the bugtracker for similar feature requests including closed ones


## Description

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Currently when downloading metadata of a DRM protected video with options like '--write-thumbnail', '--write-info-json' and '--write-description', youtube-dl will show the error 'ERROR: This video is DRM protected' and exit. Even when using the option '--skip-download'.
However, in some situations it would be useful to be able to download video metadata even though the video is protected by DRM.

Here's an example of the current situation: 
```
$ youtube-dl --skip-download --write-thumbnail --write-description "https://www.npostart.nl/binnenstebuiten/20-11-2019/KN_1710420"
[npo] KN_1710420: Downloading token
[npo] KN_1710420: Downloading player JSON
[npo] KN_1710420: Downloading hls profile JSON
[npo] KN_1710420: Downloading dash-widevine profile JSON
[npo] KN_1710420: Downloading dash-playready profile JSON
[npo] KN_1710420: Downloading smooth profile JSON
ERROR: This video is DRM protected.
```

### Relevant options
Using the following options with a DRM protected video will produce the same output as the example above: 
- --write-description
- --write-info-json
- --write-annotations
- --write-thumbnail
- --write-all-thumbnails
- --get-url
- --get-title
- --get-id
- --get-thumbnail
- --get-description
- --get-duration
- --get-filename
- --get-format
- --dump-json
- --dump-single-json
- --print-json
- --list-formats
- --write-sub
- --all-subs
- --list-subs

### Solution
There are two cases when downloading metadata from a DRM protected video:
- not using the option '--skip-download'
In this case the error about not being able to download the video due to DRM could be downgraded to a warning, with youtube-dl proceeding to download metadata. However, it may be a good idea to keep the current situation to make it very clear that the video cannot be downloaded.

- using the option '--skip-download'
In this case the error about not being able to download the video due to DRM should not stop the metadata from being downloaded. The error may be downgraded to a warning or not shown at all.