fixed some http status codes returned by some APIs

[bmpotter]

• GET /api/orgs/{ord-id}/vouchers/{device-id} and GET /api/vouchers/{device-id}
  • When device-id not found, changed http code from 400 to 404
  • When device-id is not in the client's org, changed http code from 400 to 403
• DELETE /api/orgs/{org-id}/keys/{key-name}
  • When key-name not found, changed http code from 204 to 404

Signed-off-by: Bruce Potter bp@us.ibm.com