Respond with 404 when video is not ready
Avoid directly exposing S3 video URLs

Introducing our own URLs for the MP4 and WebM formats for each video
abstracts us away from S3. Although the S3 URL is still discoverable,
we would expect people to use these new URLs.
Avoid directly exposing S3 poster image URLs

Introducing our own URLs for the poster image for each video abstracts
us away from S3. Although the S3 URL is still discoverable, we would
expect people to use these new URLs.