provide clearer error message when oembed is disabled

[DanielDent]

When a YouTube video has embedding disabled, the oembed endpoint returns a value to indicate that the request is unauthorized. It would be helpful if that did not result in the same {:error, "oEmbed not found"} return value that occurs for invalid URLs as well -- being able to tell the end-user "this is a valid video URL but it has embedding disabled" would be preferable.