You can edit the constant DefaultLoginPage. You can specify a different login page here for when the login process fails. See also chapter 2 of https://docs.mendix.com/appstore/modules/saml/
If that doesn't work for you: another thing you can do is add custom CSS to the elements on this page. You can use this to hide the logo, for example:
.saml-container > .login-form > .login-logo {
display: none;
}
You can even use the before and after to edit the content. https://developer.mozilla.org/en-US/docs/Web/CSS/::before
Hope this helps!