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/security/qa/security-trial/src/javaRestTest/java/org/elasticsearch/xpack/security/apikey/ApiKeyRestIT.java
Map.entry("username", END_USER),
            Map.entry("password", END_USER_PASSWORD.toString()),
            Map.entry("api_key", Map.of("name", "test_api_key_password"))
        );
        request.setJsonEntity(XContentTestUtils.convertToXContent(requestBody, XContentType.JSON).utf8ToString());
        final Response response = client().performRequest(request);
        final Map<String, Object> responseBody = entityAsMap(response);
        assertThat(responseBody.get("name"), equalTo("test_api_key_password"));
        assertThat(responseBody.get("id"), notNullValue());
        assertThat(responseBody.get("id"), instanceOf(String.class));
        ApiKey apiKey = getApiKey((String) responseBody.get("id"));
        assertThat(apiKey.getUsername(), equalTo(END_USER));
    }
    public void testGrantApiKeyForOtherUserWithAccessToken() throws IOException {
        final Tuple<String, String> token = super.createOAuthToken(END_USER, END_USER_PASSWORD);
        final String accessToken = token.v1();
        final Request request = new Request("POST", "_security/api_key/grant");
        request.setOptions(
            RequestOptions.DEFAULT.toBuilder()
                .addHeader("Authorization", UsernamePasswordToken.basicAuthHeaderValue(SYSTEM_USER, SYSTEM_USER_PASSWORD))
        );
        final Map<String, Object> requestBody = Map.ofEntries(
            Map.entry("grant_type", "access_token"),
            Map.entry("access_token", accessToken),
            Map.entry("api_key", Map.of("name", "test_api_key_token", "expiration", "2h"))
        );
        request.setJsonEntity(XContentTestUtils.convertToXContent(requestBody, XContentType.JSON).utf8ToString());
        final Instant before = Instant.now();
        final Response response = client().performRequest(request);
        final Instant after = Instant.now();