Error response format change

Our current error response format defined in `api.md` doesn't cooperate with hapi very well.

Default hapi output looks like:

```json
{
  "code": 400,
  "error": "Bad Request",
  "message": "Validation error, blah blah..."
}
```
It nicely responds to validation errors (for example) without any app code assistance.

`api.md` clashes a bit with this format because `error` becomes a number. I propose we just add an `errno` instead for application level error numbers.

Not a big change, just thought I'd bring it up.