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
protected boolean extendedCheck(String action, TransportRequest request, Authentication authentication) {
            if (request instanceof CreateApiKeyRequest) {
                return true;
            } else if (request instanceof final GetApiKeyRequest getApiKeyRequest) {
                return checkIfUserIsOwnerOfApiKeys(
                    authentication,
                    getApiKeyRequest.getApiKeyId(),
                    getApiKeyRequest.getUserName(),
                    getApiKeyRequest.getRealmName(),
                    getApiKeyRequest.ownedByAuthenticatedUser()
                );
            } else if (request instanceof final InvalidateApiKeyRequest invalidateApiKeyRequest) {
                final String[] apiKeyIds = invalidateApiKeyRequest.getIds();
                if (apiKeyIds == null) {
                    return checkIfUserIsOwnerOfApiKeys(
                        authentication,
                        null,
                        invalidateApiKeyRequest.getUserName(),
                        invalidateApiKeyRequest.getRealmName(),
                        invalidateApiKeyRequest.ownedByAuthenticatedUser()
                    );
                } else {
                    return Arrays.stream(apiKeyIds)
                        .allMatch(
                            id -> checkIfUserIsOwnerOfApiKeys(
                                authentication,
                                id,
                                invalidateApiKeyRequest.getUserName(),
                                invalidateApiKeyRequest.getRealmName(),
                                invalidateApiKeyRequest.ownedByAuthenticatedUser()