Review Register error code documentation

[nils-work]

Description

As noted in the Future improvements section, the Register error responses are currently inconsistent with the errors defined for other endpoints.

The changes may only require documentation updates, but the specification may need to be reviewed against the implementation to clarify whether the changes would cause a breaking change.

Intention and Value of Change

To provide accurate and consistent error handling documentation.

Area Affected

Error responses for endpoints in the Register APIs section.

Change Proposed

Resolve the details in the Future improvements section.

The Register endpoints should be changed from the generic error details -

to the format of other endpoints, for example -