Update the handling of Vend API Errors

It turns out that the error_description attribute on the Vend error
responses is option. Add some checking to ensure that the
error_description attribute is only used if it exists.

Author: chriswoodford

lib/Vend/Api/AuthenticationGateway.php

@@ -206,7 +206,13 @@ protected function requestAccess($request)
         ));
 
         if (isset($response->error)) {
-            throw new \RuntimeException($response->error_description);
+            $message = $response->error;
+
+            if (isset($response->error_description)) {
+                $message = $response->error_description;
+            }
+
+            throw new \RuntimeException($message);
         }
 
         return $response;