message: docs: add more description at `Error Handling`
diff: @@ -80,16 +80,18 @@ If you omit `auth` parameter, the client uses Session authentication.
  ## Error Handling
  -When the API request responds with a status code other than 200, the client raises [`KintoneRestAPIError`](src/KintoneRestAPIError.ts)
  +When the API request responds with a status code other than 200, the client raises [`KintoneRestAPIError`](src/KintoneRestAPIError.ts).
  +
  +[`KintoneRestAPIError`](src/KintoneRestAPIError.ts) has the following properties:
  | Name | Type | Description |
  -| ---------------- | :----: | -------------------------------------------------------------------------------------------------- |
  +| ---------------- | :-------------------: | ---------------------------------------------------------------------------------------------------------------------------------------------------------- |
  | id | String | The ID of the error. |
  | code | String | The code of the error, to specify the type of error it is. |
  | status | Number | The HTTP status of the response. |
  | headers | Object | The HTTP headers of the response. |
  | message | String | The error message. |
  -| bulkRequestIndex | Number | When executing [bulkRequest](docs/bulkRequest.md) and gets failed response, this index responds to |
  +| bulkRequestIndex | Number or `undefined` | The index of the failed request when executing [bulkRequest](docs/bulkRequest.md) and one of the requests fails.<br />This value is `undefined` otherwise. |
  ## References
  