No Error when a invalid domain key is used

[tomfotherby]

Here is a test case:

    // Set a incorrect domain key
    Apostle::setup("incorrect_key_entered");

    // Create a mail instance
    $mail = new Mail( "welcome", ["email" => "tom@example.com" ] );

    // Add any values our template needs
    $mail->username = "Snikch";

    // Send to Apostle.io
    $result = $mail->deliver($failureMsg);

    echo $result ? "all ok" : "Something failed: $failureMsg\n";

It prints "all ok". Would be better if we could get a error message saying domain key is incorrect.