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
I'd vote for
```
res.status code or 400
res.send new Buffer...
```