When Drupal attempts to enroll a user in a closed course, a 406 will
be returned. This causes the marketing site to redirect to the track
selection page for that course, which will then redirect to the
dashboard with a nice message.
ECOM-2317
These endpoints are currently for internal use only, but should be versioned nonetheless; Drupal will begin using the basket creation endpoint soon. No functionality has been changed. XCOM-494.
