Fix #69 issue by checking if response structure contains a "error" attribute

Author: leganz

src/Exceptions/CouldNotSendNotification.php

@@ -10,6 +10,10 @@ class CouldNotSendNotification extends \Exception
      */
     public static function serviceRespondsNotSuccessful($response)
     {
+        if (isset($response->error)){
+            return new static("Couldn't post notification. Response: ".$response->error);
+        }
+
         $responseBody = print_r($response->errors[0]->message, true);
 
         return new static("Couldn't post notification. Response: ".$responseBody);