Placeholder for handling EME key status "output-downscaled"

[robwalch]

This PR will...

Handle EME key status "output-downscaled"

Why is this Pull Request needed?

Either another ErrorDetails should be defines, similar to KEY_SYSTEM_STATUS_OUTPUT_RESTRICTED. Or, additional fields should accompany the error for determining if HDCP Level or other down-switch actions (resolution/bitrate/audio-channels cap) should be used to resolve the issue.

Are there any points in the code the reviewer needs to double check?

Resolves issues:

Checklist

• [ ] changes have been done against master branch, and PR does not conflict
• [ ] new unit / functional tests have been added (whenever applicable)
• [ ] API or design changes are documented in API.md