clean output buffer for Throwable instead of just Exception (#797)

Fixes #514

libs/sysplugins/smarty_internal_templatebase.php

@@ -257,7 +257,7 @@ abstract class Smarty_Internal_TemplateBase extends Smarty_Internal_Data
                 error_reporting($_smarty_old_error_level);
             }
             return $result;
-        } catch (Exception $e) {
+        } catch (Throwable $e) {
             while (ob_get_level() > $level) {
                 ob_end_clean();
             }