Make exception handling experience intuitive

I'm encoutering an issue with using ApiException to get exceptions in v2.0.0 RC9. The message variable is always blank with exceptions for most commands regarding working with sites, drives, and files. The exception is in there, just buried inside of a private 'error' variable.

Example of getting a site that doesn't exist:

<?php

error_reporting(E_ALL);
ini_set('display_errors', '1');

require_once __DIR__ . '/vendor/autoload.php';

use Microsoft\Graph\Generated\Models;
use Microsoft\Graph\GraphRequestAdapter;
use Microsoft\Graph\GraphServiceClient;
use Microsoft\Kiota\Authentication\Oauth\ClientCredentialContext;
use Microsoft\Kiota\Authentication\PhpLeagueAuthenticationProvider;

try {
    $tokenRequestContext = new ClientCredentialContext(
        $tenantId,
        $clientId,
        $clientSecret
    );
    $scopes = ['https://graph.microsoft.com/.default'];
    $authProvider = new PhpLeagueAuthenticationProvider($tokenRequestContext, $scopes);
    $requestAdapter = new GraphRequestAdapter($authProvider);
    $graphServiceClient = new GraphServiceClient($requestAdapter);

    /** @var Models\Site $site  */
    $site = $graphServiceClient->sitesById($siteId)->get()->wait();
} catch (ApiException $ex) {
    echo $ex->getMessage();
}

?>

This returns the a blank $ex->message, and the following var_dump($ex):

object(Microsoft\Graph\Generated\Models\ODataErrors\ODataError)#59 (9) {
  ["message":protected]=>
  string(0) ""
  ["string":"Exception":private]=>
  string(0) ""
  ["code":protected]=>
  int(0)
  ["file":protected]=>
  string(131) "/home/username/test/vendor/microsoft/microsoft-graph/src/Generated/Models/ODataErrors/ODataError.php"
  ["line":protected]=>
  int(37)
  ["trace":"Exception":private]=>
  array(7) {
    [0]=>
    array(5) {
      ["file"]=>
      string(114) "/home/username/test/vendor/microsoft/kiota-serialization-json/src/JsonParseNode.php"
      ["line"]=>
      int(107)
      ["function"]=>
      string(28) "createFromDiscriminatorValue"
      ["class"]=>
      string(55) "Microsoft\Graph\Generated\Models\ODataErrors\ODataError"
      ["type"]=>
      string(2) "::"
    }
    [1]=>
    array(5) {
      ["file"]=>
      string(114) "/home/username/test/vendor/microsoft/kiota-http-guzzle/src/GuzzleRequestAdapter.php"
      ["line"]=>
      int(308)
      ["function"]=>
      string(14) "getObjectValue"
      ["class"]=>
      string(48) "Microsoft\Kiota\Serialization\Json\JsonParseNode"
      ["type"]=>
      string(2) "->"
    }
    [2]=>
    array(5) {
      ["file"]=>
      string(114) "/home/username/test/vendor/microsoft/kiota-http-guzzle/src/GuzzleRequestAdapter.php"
      ["line"]=>
      int(88)
      ["function"]=>
      string(19) "throwFailedResponse"
      ["class"]=>
      string(41) "Microsoft\Kiota\Http\GuzzleRequestAdapter"
      ["type"]=>
      string(2) "->"
    }
    [3]=>
    array(5) {
      ["file"]=>
      string(99) "/home/username/test/vendor/php-http/promise/src/FulfilledPromise.php"
      ["line"]=>
      int(35)
      ["function"]=>
      string(30) "Microsoft\Kiota\Http\{closure}"
      ["class"]=>
      string(41) "Microsoft\Kiota\Http\GuzzleRequestAdapter"
      ["type"]=>
      string(2) "->"
    }
    [4]=>
    array(5) {
      ["file"]=>
      string(114) "/home/username/test/vendor/microsoft/kiota-http-guzzle/src/GuzzleRequestAdapter.php"
      ["line"]=>
      int(98)
      ["function"]=>
      string(4) "then"
      ["class"]=>
      string(29) "Http\Promise\FulfilledPromise"
      ["type"]=>
      string(2) "->"
    }
    [5]=>
    array(5) {
      ["file"]=>
      string(135) "/home/username/test/vendor/microsoft/microsoft-graph/src/Generated/Sites/Item/SiteItemRequestBuilder.php"
      ["line"]=>
      int(277)
      ["function"]=>
      string(9) "sendAsync"
      ["class"]=>
      string(41) "Microsoft\Kiota\Http\GuzzleRequestAdapter"
      ["type"]=>
      string(2) "->"
    }
    [6]=>
    array(5) {
      ["file"]=>
      string(64) "/home/username/test/graph-bug.php"
      ["line"]=>
      int(32)
      ["function"]=>
      string(3) "get"
      ["class"]=>
      string(59) "Microsoft\Graph\Generated\Sites\Item\SiteItemRequestBuilder"
      ["type"]=>
      string(2) "->"
    }
  }
  ["previous":"Exception":private]=>
  NULL
  ["additionalData":"Microsoft\Graph\Generated\Models\ODataErrors\ODataError":private]=>
  array(0) {
  }
  ["error":"Microsoft\Graph\Generated\Models\ODataErrors\ODataError":private]=>
  object(Microsoft\Graph\Generated\Models\ODataErrors\MainError)#80 (6) {
    ["additionalData":"Microsoft\Graph\Generated\Models\ODataErrors\MainError":private]=>
    array(1) {
      ["innerError"]=>
      array(3) {
        ["date"]=>
        string(19) "2022-10-17T16:23:06"
        ["request-id"]=>
        string(36) "80ddeda1-e38d-41fc-a42d-6e943d3b16cd"
        ["client-request-id"]=>
        string(36) "7ee8f8ec-2bd6-4609-aa90-3a2356efa61f"
      }
    }
    ["code":"Microsoft\Graph\Generated\Models\ODataErrors\MainError":private]=>
    string(12) "itemNotFound"
    ["details":"Microsoft\Graph\Generated\Models\ODataErrors\MainError":private]=>
    NULL
    ["innererror":"Microsoft\Graph\Generated\Models\ODataErrors\MainError":private]=>
    NULL
    ["message":"Microsoft\Graph\Generated\Models\ODataErrors\MainError":private]=>
    string(33) "Requested site could not be found"
    ["target":"Microsoft\Graph\Generated\Models\ODataErrors\MainError":private]=>
    NULL
  }
}

Start with Generated/Models/ODataErrors/ODataError.php and MainError, then trace the failure through GuzzleRequestAdapter::throwFailedResponse and SiteItemRequestBuilder.php. Reproduce the missing message with the sitesById(...)->get()->wait() example. Done means the API error message is available through ApiException::getMessage().