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
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0; you may not use this file except in compliance with the Elastic License
 * 2.0.
 */
package org.elasticsearch.xpack.core.security.authz.privilege;
import org.elasticsearch.common.Strings;
import org.elasticsearch.transport.TransportRequest;
import org.elasticsearch.xpack.core.security.action.apikey.CreateApiKeyRequest;
import org.elasticsearch.xpack.core.security.action.apikey.GetApiKeyRequest;
import org.elasticsearch.xpack.core.security.action.apikey.InvalidateApiKeyRequest;
import org.elasticsearch.xpack.core.security.action.apikey.QueryApiKeyRequest;
import org.elasticsearch.xpack.core.security.authc.Authentication;
import org.elasticsearch.xpack.core.security.authc.AuthenticationField;
import org.elasticsearch.xpack.core.security.authc.RealmDomain;
import org.elasticsearch.xpack.core.security.authz.permission.ClusterPermission;
import org.elasticsearch.xpack.core.security.support.Automatons;
import java.util.Arrays;
/**
 * Named cluster privilege for managing API keys owned by the current authenticated user.
 */
public class ManageOwnApiKeyClusterPrivilege implements NamedClusterPrivilege {