"type": "MissingParameterError" for existing users (POST to @users)

Trying to create a new user (POST to @users) creates the following error (which is ok) if a user already exists. However "type": "MissingParameterError" is misleading and makes no sense here.

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "migration/migrate.py", line 360, in <module>
    migrate_users()
  File "migration/migrate.py", line 351, in migrate_users
    add_user(old2new_path('dynamore'), d.id, pw, fullname, email, roles)
  File "migration/migrate.py", line 172, in add_user
    raise RuntimeError(result.text)
RuntimeError: {
  "error": {
    "message": "The login name you selected is already in use or is not valid. Please choose another.",
    "type": "MissingParameterError"
  }
}

Start with the POST to @users described in migration/migrate.py, especially the add_user call reported at line 172, and reproduce the existing-user response. Trace where that response's error type is assigned; done means the response keeps the existing-user message while reporting an error type that accurately describes the failure.