For clarity (and sort of to review my fix) I'll explain what I think the issue
meant (not entirely clear) and how to test:
When going to a Silva edit URL that requires logging in, when pressing 'cancel'
one would see the Zope standard error message, stating that not only did you not
provide proper credentials, but also there's an error in the error page that
should have been displayed. I have changed the error page so it does get
displayed correctly, if people click 'cancel' now they get a page that explains
you need credentials and asks if you want to retry, all in nice Silva style.
To test, make sure you install the new standard_error_message page (by removing
it and pressing 'install default layout' on service_extensions), go to an edit
url (e.g. '/silva/edit') and press 'cancel' when the browser asks for your