Add css class on exist error

[cTahirih]

In order to be able to change styles in case of an error, it would be convenient to have a variable to handle this state such as

[AlexMiniApps]

@cTahirih Hello, ok. I will add it in the features list. Right now you can try to use the following technics: https://angular.io/guide/component-styles#deprecated-deep--and-ng-deep