Users with only `manage_own_api_key` privilege is not allowed to use the [GrantApiKey API](https://www.elastic.co/guide/en/elasticsearch/reference/current/security-api-grant-api-key.html#security-api-grant-api-key). The overall behaviour is correctly implemented but the error message could use some improvements. Currently it is something like the follows:

```
POST _security/api_key/grant
{
  "password": "password",
  "grant_type": "password",
  "username": "foo",
  "api_key": {
    "name": "k1"
  }
}

===
{
  "error": {
    "root_cause": [
      {
        "type": "illegal_argument_exception",
        "reason": "manage own api key privilege only supports API key requests (not org.elasticsearch.xpack.core.security.action.apikey.GrantApiKeyRequest)"
      }
    ],
    "type": "illegal_argument_exception",
    "reason": "manage own api key privilege only supports API key requests (not org.elasticsearch.xpack.core.security.action.apikey.GrantApiKeyRequest)"
  },
  "status": 400
}
```

This is different from other unauthorized error messages which should be along the line of 
> action [..] is unauthorized for user [...] with roles [...], this action is granted by ..

The status code should also be `403` instead of `400`.
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/privilege/ManageOwnApiKeyClusterPrivilege.java
)
                        );
                }
            } else if (request instanceof final QueryApiKeyRequest queryApiKeyRequest) {
                return queryApiKeyRequest.isFilterForCurrentUser();
            }
            throw new IllegalArgumentException(
                "manage own api key privilege only supports API key requests (not " + request.getClass().getName() + ")"
            );
        }
        @Override
        protected boolean doImplies(ClusterPermission.ActionBasedPermissionCheck permissionCheck) {
            return permissionCheck instanceof ManageOwnClusterPermissionCheck;
        }
        private static boolean checkIfUserIsOwnerOfApiKeys(
            Authentication authentication,
            String apiKeyId,
            String username,
            String realmName,
            boolean ownedByAuthenticatedUser
        ) {
            if (isCurrentAuthenticationUsingSameApiKeyIdFromRequest(authentication, apiKeyId)) {
                return true;
            } else {
                /*
                 * TODO bizybot we need to think on how we can propagate appropriate error message to the end user when username, realm name
                 *   is missing. This is similar to the problem of propagating right error messages in case of access denied.
                 */
                if (authentication.isApiKey()) {