Add description of error response body format

# Authencation

```json
{
  "error_description": "[PR400-AN-0016] - Required parameter [grant_type] missing.",
  "error": "invalid_request"
}
```

# Others

```json
{
  "code": "PR405-MC-0001",
  "message": {
    "lang": "en",
    "value": "Method not allowed."
  }
}
```