Some errors missing required field?

[googleson78]

For most of the errors, we have the following required fields:

required:
  - item
  - error
  - error_description
  - error_details

However, for some of them (response/error_unregistered.yaml and response/error_already_registered.yaml), the error_details field is not required. This seems a bit weird, so I have slight suspicions that this is not an intentional omission. Is this the case? Sorry for the trouble otherwise. If it is omitted intentionally, it might be worth documenting why (unless it's also documented somewhere and I missed it)