form based authentication is not part of struts. It's a container-specific functionality. Tomcat for instance can be configured to use form-based authentication, with which it will redirect requests to your login form if it doesn't recognise the request as part of an authenticated session. Doesn't matter if the request is for the struts servlet or an error - it depends on how you configure the authentication options in your appserver.
I have seen things you people would not believe, attack ships on fire off the shoulder of Orion, c-beams sparkling in the dark near the Tennhauser Gate. All these moments will be lost in time, like tears in the rain.