Confusing error if user tries to use a YouTube or Vimeo video with the Canvas Studio integration

[robertknight]

The Canvas Studio integration currently only supports videos that have been directly uploaded to Canvas Studio, not videos from YouTube or Vimeo which have been added to the user's Canvas Studio library.

At present the integration will allow the user to create a video with a YouTube / Vimeo video from the library, but launching fails with a confusing error (422 status code).

Ideally we would determine the problem at the point when the assignment is created. If that isn't possible, we should at least handle the issue more gracefully when launched.

See https://hypothes-is.slack.com/archives/C2BLQDKHA/p1714406382388799.

[robertknight]

See also #6194.