Add default error message

Github also throws `custom` error messages and the switch doesn't catch it.

```[errors] => Array
        (
            [0] => Array
                (
                    [code] => custom
                    [resource] => Repository
                    [field] => name
                    [message] => name can't be private. You are over your quota.
                )




        )```

Adding a `default` fixes this.

Author: m4tthumphrey

lib/Github/HttpClient/Listener/ErrorListener.php

@@ -71,10 +71,14 @@ public function postSend(RequestInterface $request, MessageInterface $response)
                                 $errors[] = sprintf('Field "%s" already exists, for resource "%s"', $error['field'], $error['resource']);
                                 break;
 
+                            default:
+                                $errors[] = $error['message'];
+                                break;
+
                         }
                     }
 
-                    throw new ValidationFailedException('Validation Failed:' . implode(', ', $errors), 422);
+                    throw new ValidationFailedException('Validation Failed: ' . implode(', ', $errors), 422);
                 }
             }