Access control with login-form on the same page

[lpa]

I have defined a login-handler shown below to print the login-form on each page that requires access when the user is not logged in and is under some parent-page:

if ($page->parents->has('name=memberpage') { $section = 'memberpage'; }

if(!$user->isLoggedin() and $section != '') {
  # print the login form
} elseif ($user->isLoggedin() and restrict_content($section)) {
  # user doesn't belong to the required group
}

This way I can show login-form on the same page that is restricted and not redirect the user to a separate login-page. 

Now I tried to use CustomPageRoles module for simple access control. But the module redirects me to the 404-page when I don't have access, when not logged in.

How could I just check whether to print the login form or show the page with CustomPageRoles handling the access control roles? Can I somehow read with the api that the user doesn't have access to the page without redirection to 404?