XF\Error should be extendable

[DragonByte Tech]

Function calls like $exceptionMessage = $this->adjustExceptionMessage($e->getMessage(), $e); with the code
PHP:

    protected function adjustExceptionMessage($message, $e)
    {
        return $message;
    }

Implies that XF\Error should be extendable, but it isn't:
PHP:

        $container['error'] = function ()
        {
            return new Error($this);
        };