Fix Dashboard Nav Bookmarks

Fix Dashboard Navigation Bookmarks

- Check if page exists before adding item
- Remove item from User Config if page no longer exists
- Switch `is_object()` to `isError()` as `Page::getByID()` always
  returns a Page object

www.concrete5.org/developers/bugs/5-6-1-2/dashboard-edit-button-hover-no-popup-bug.../