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/test/java/org/elasticsearch/xpack/core/security/authz/privilege/ManageOwnApiKeyClusterPrivilegeTests.java
).runAs(new User("user_b"), new Authentication.RealmRef("realm_b", "realm_b_type", randomAlphaOfLengthBetween(3, 8)));
        assertTrue(
            clusterPermission.check(
                "cluster:admin/xpack/security/api_key/get",
                GetApiKeyRequest.usingRealmAndUserName("realm_b", "user_b"),
                authentication
            )
        );
        assertTrue(
            clusterPermission.check(
                "cluster:admin/xpack/security/api_key/invalidate",
                InvalidateApiKeyRequest.usingRealmAndUserName("realm_b", "user_b"),
                authentication
            )
        );
    }
    public void testCheckQueryApiKeyRequest() {
        final ClusterPermission clusterPermission = ManageOwnApiKeyClusterPrivilege.INSTANCE.buildPermission(ClusterPermission.builder())
            .build();
        final QueryApiKeyRequest queryApiKeyRequest = new QueryApiKeyRequest();
        if (randomBoolean()) {
            queryApiKeyRequest.setFilterForCurrentUser();
        }
        assertThat(
            clusterPermission.check(QueryApiKeyAction.NAME, queryApiKeyRequest, AuthenticationTestHelper.builder().build()),
            is(queryApiKeyRequest.isFilterForCurrentUser())
        );
    }
}