Fix fetching initial playlist data

[dnicolson]

Description

YouTube appears to have made some changes again, the response is an object instead of an array. The unused PlaylistDataFetchError error has also been repurposed.

How Has This Been Tested?

In Violentmonkey after building.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[dnicolson]

The response has now changed back.