label: Supplement IssueLabelApp's request handling

* Return 406 to a request for labels, if the client does not accept
  application/json.
* Return 403 to a request for labels, if the user does not have
  permission to access the project owns the labels.
* Add descriptive content bodies for 4xx errors.
* Rename the parameter 'userName' to 'owerName' to make it clear.
* Do not contain body in a response to a request for adding a label, if
  the client does not accept application/json.