Catch error not being set on response data array

Author: chris-ware

src/Provider/Xero.php

@@ -130,7 +130,7 @@ protected function checkResponse(ResponseInterface $response, $data)
     {
         if ($response->getStatusCode() >= 400) {
             throw new XeroProviderException(
-                $data['error'] ?: $response->getReasonPhrase(),
+                isset($data['error']) ? $data['error'] : $response->getReasonPhrase(),
                 $response->getStatusCode(),
                 $response
             );