fix(platform/api): catch DoesNotExist in quiz question/answer update to return 400 instead of 500

[payamnj]

Problem

During nested quiz update in SingleCourseContentView._update_course_content_atomic, the code calls .get(id=...) on question and answer querysets without error handling:

• quiz.questions.get(id=question_data.id)
• question.answers.get(id=answer_data.id)

A stale or cross-quiz ID raises DoesNotExist which results in a 500 response instead of a structured 400/404.

Suggestion

Catch DoesNotExist for both lookups and return a validation-style error response (400 with a descriptive message).