Display more informative error message for closed courses

[cliu587]

When users attempt to use clickstream export with the command
courseraresearchexports jobs request tables --course_slug COURSE_SLUG ... when COURSE_SLUG is a closed course, we try to translate the course slug to course id using https://www.coursera.org/api/onDemandCourses.v1/?q=slug&slug=COURSE_SLUG.

We use the unauthenticated version of this API because we do not have access to the user's Coursera cookies when making this request. For closed courses, this request returns an error.

We should document this potential issue and provide an alternative (hit the API in your browser, and request with courseraresearchexports jobs request tables --course_id ...).

Ideally we will be able to open the above API with OAuth2 as well and not need this workaround.