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
import org.elasticsearch.client.ResponseException;
import org.elasticsearch.common.settings.SecureString;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.test.XContentTestUtils;
import org.elasticsearch.xcontent.XContentType;
import org.elasticsearch.xpack.core.security.action.apikey.ApiKey;
import org.elasticsearch.xpack.core.security.authc.support.UsernamePasswordToken;
import org.elasticsearch.xpack.security.SecurityOnTrialLicenseRestTestCase;
import org.junit.After;
import org.junit.Before;
import java.io.IOException;
import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.util.List;
import java.util.Map;
import java.util.Set;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.emptyString;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.greaterThanOrEqualTo;
import static org.hamcrest.Matchers.hasEntry;
import static org.hamcrest.Matchers.instanceOf;
import static org.hamcrest.Matchers.lessThanOrEqualTo;
import static org.hamcrest.Matchers.not;
import static org.hamcrest.Matchers.notNullValue;
/**
 * Integration Rest Tests relating to API Keys.
 * Tested against a trial license
 */
public class ApiKeyRestIT extends SecurityOnTrialLicenseRestTestCase {