Deliver metadata with business errors

[defagos]

Description

This PR makes it possible to deliver metadata event when a business error prevents playback. This makes it possible to create more engaging experiences in this case (e.g. display content information or thumbnail).

Changes made

• Add asset type that carries both an error and metadata.
• The demo player layout was not updated. It is complicated enough already.

Checklist

• [x] APIs have been properly documented (if relevant).
• [x] The documentation has been updated (if relevant).
• [x] New unit tests have been written (if relevant).
• [x] The demo has been updated (if relevant).