__toString exception workaround

timber-function-wrapper.php

    public function __toString() {
       try {
           return $this->call();
       } catch(\Exception $e) {
               // the __toString method isn't allowed to throw exceptions
               // so we turn them into an error instead
               trigger_error($e->getMessage() . "\n" . $e->getTraceAsString(), E_USER_ERROR);
               return '';
           }
    }