Nesse código, tento executar uma seleção em uma tabela que não existe,getJobReference() retornaNULL e adoraria pegar esse tipo de erro e gostaria de receber mensagens de erro de alguma forma.

Como obter a mensagem de erro quando algo falha?

$client = new Google_Client();
$client->setApplicationName("Client_Library_Examples");
//$client->setDeveloperKey(API_KEY);

if (isset($_SESSION['service_token'])) {
    $client->setAccessToken($_SESSION['service_token']);
}
$key = file_get_contents($key_file_location);
$cred = new Google_Auth_AssertionCredentials(
        $service_account_name, array(
    'https://www.googleapis.com/auth/bigquery',
        ), $key
);
$client->setAssertionCredentials($cred);
if ($client->getAuth()->isAccessTokenExpired()) {
    $client->getAuth()->refreshTokenWithAssertion($cred);
}
$_SESSION['service_token'] = $client->getAccessToken();
var_dump($_SESSION);
$bq = new Google_Service_Bigquery($client);

//build query
$sql = 'select * from example.table LIMIT 10';

$job = new Google_Service_Bigquery_Job();
$config = new Google_Service_Bigquery_JobConfiguration();
$queryConfig = new Google_Service_Bigquery_JobConfigurationQuery();
$config->setQuery($queryConfig);

$job->setConfiguration($config);
$queryConfig->setQuery($sql);

$insert = new Google_Service_Bigquery_Job($bq->jobs->insert(PROJECT_ID, $job));
$jr = $insert->getJobReference();
var_dump($jr);/*THIS RETURNS NULL */
$jobId = $jr['jobId'];

$res = new Google_Service_Bigquery_GetQueryResultsResponse($bq->jobs->getQueryResults(PROJECT_ID, $jobId));

//see the results made it as an object ok:
var_dump($res);