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
public static final ManageOwnApiKeyClusterPrivilege INSTANCE = new ManageOwnApiKeyClusterPrivilege();
    private static final String PRIVILEGE_NAME = "manage_own_api_key";
    private final ClusterPermission permission;
    private ManageOwnApiKeyClusterPrivilege() {
        permission = this.buildPermission(ClusterPermission.builder()).build();
    }
    @Override
    public String name() {
        return PRIVILEGE_NAME;
    }
    @Override
    public ClusterPermission.Builder buildPermission(ClusterPermission.Builder builder) {
        return builder.add(this, ManageOwnClusterPermissionCheck.INSTANCE);
    }
    @Override
    public ClusterPermission permission() {
        return permission;
    }
    private static final class ManageOwnClusterPermissionCheck extends ClusterPermission.ActionBasedPermissionCheck {
        public static final ManageOwnClusterPermissionCheck INSTANCE = new ManageOwnClusterPermissionCheck();
        private ManageOwnClusterPermissionCheck() {
            super(Automatons.patterns("cluster:admin/xpack/security/api_key/*"));
        }
        @Override