Don't allow API calls with sort=random + last_id

[jtotoole]

Post-Colorado Mystery, make the API return an error when these parameters are used together in calls to stories/list endpoint: sort=random and processed_stories_id>0.

[rahulbot]

(note: I added a check to prevent this within the API client, which gets us a good bit of the way there because it stops ourselves and our tools from returning results if we try to do this)