OpenAPI `Error.code` schema property SHOULD reference `ErrorCode` schema

This requires #19 to be completed first.

```yaml
Error:
  type: object
  required: [id, kind, href, code, reason]
  properties:
    id:
      type: string
    kind:
      type: string
    href:
      type: string
    code:
      $ref: '#/components/schemas/ErrorCode'
    reason:
      type: string

```