[core] Make a specific function for Page::display_error() so that themes can override it

[shish]

The default implementation of display_error() being to remove any existing Blocks, add a new Block with an error message, and then do the same stuff that display_page() does

    protected function display_error(): void
    {
        $error = $this->error;
        assert($error !== null);
        $this->set_code($error->http_code);
        $this->set_title("Error");
        $this->blocks = [];
        $this->add_block(new Block(null, \MicroHTML\SPAN($error->getMessage())));
        $this->display_page();
    }