Assert the page is (or is not) the "Access denied" or the "Page not found" page of the site: its heading and the HTTP status of its address for the session of the browser (403 or 404).
Examples
Then the page should be an access denied page
Then the page should be a page not found page
And the page should not be an access denied page
Then the page should not be a page not found page
And the page should be an access denied page
Pattern
Then(/^the page should( not)* be an? (access denied|page not found) page$/, …)
Part of @webship-js/backdrop-cms-steps (github.com/webship/backdrop-cms-steps), a step pack for Webship-js 2.0.x.