if error exists, returns given string.

[asaokamei]

Nice to have a way to simplify the following code,

    $forms->formGroup(
        $forms->text('jumper', $view->data->raw('jumped'))->id(),
    )->class($view->errors->exists('jumped') ? 'has-error' : null);

to such as

    $forms->formGroup(
        $forms->text('jumper', $view->data->raw('jumped'))->id(),
    )->class($view->errors->ifSet('jumped', 'has-error'));