Tests for error handling and related (minor™) fixes

Part of

- https://trello.com/c/CuPDy6Ve/1739-api-blueprint-api-error-handling-proper-http-codes
- https://trello.com/c/ObThuTMy/1755-proper-testsuite-for-api-blueprint-api-app
Didn't get one :-o Otherwise I'd vote for it too :) :+1: