recentrated: check if source profile is private

[andre-st]

Cannot load shelf if registered user is not public (to search engines) but visible to Goodreads users only. Could set_good_cookie_file() but I like this program to run without my cookie. Should add a check in extract_books() which looks for: "This Profile Is Restricted to Goodreads Users. [...] Sign in to Goodreads to Learn More About $USER." Perhaps, send an warning email to user?