REST endpoints, associated Porter Learner functions, and think about failure HTTP codes.

Draft API provided by #2667 . Can re-iterate here if required, but I'll just summarize - see #2667 for full context.
For Bob::retrieve:

• /get_treasure_map - Scope Porter functionality needed by Bob #2667 (comment)
• /exec_work_order - Scope Porter functionality needed by Bob #2667 (comment)