Bug: Course Blocks API 500s for an enrolled learner when the course start date is in the future

[bmtcril]

GET /api/courses/v1/blocks/?course_id=<key>&username=<learner>&depth=all
answers HTTP 500 with the LMS error page when the learner is enrolled in a
course whose start date has not passed. The log shows KeyError: 'self_paced'
then AttributeError: Field self_paced does not exist, raised from the
course-blocks transformers reading the course block's fields.

Every neighbouring learner endpoint handles the same state cleanly:

• course_home/course_metadata returns
  course_access: {has_access: false, error_code: "course_not_started"}
• course_home/v1/outline and /navigation answer
  403 {"detail": "Course has not started"}
• courseware/sequence/<id> answers 404
• the same Blocks API call as global staff answers 200

Moving the course start into the past makes the learner's call answer 200.

Expected: answer the not-started case the way the course-home APIs do, with
a 403 carrying the access error, rather than reaching the transformers with a
course whose fields were never collected.