This PR will...

1. Handles #EXTM3U playlist parsing errors according to the request type (manifest vs level parsing error)
2. Removes some redundant playlist parsing error handling ('#EXT-X-TARGETDURATION is required' > 'Missing Target Duration')

Why is this Pull Request needed?

1. Fixes Live reload "giving up" after an empty response because it is handles as a manifest (multivariant playlist) error rather than a level (media playlist) error.
2. Removes unused '#EXT-X-TARGETDURATION is required' error message and redundant code from src

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

Resolves issues:

Fixes #7531

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