Updates the API error response when there is an unknown or
deactivated user in the `principals` parameter for either the
`/api/subscribe` or `/api/unsubscribe` endpoints. We now use
the `access_user_by_email` and `access_user_by_id` code paths,
which return an HTTP response of 400 and a "BAD_REQUEST" code.
Previously, an HTTP response of 403 was returned with a special
"UNAUTHORIZED_PRINCIPAL" code in the error response. This code
was not documented in the API documentation and is removed as
a potential JsonableError code with these changes.
Fixes#26593.
ebfe9637c8