It seems Google has removed publicized “Enhance 404 Pages” Widget from Google Webmaster Tools account.
I was looking for it everywhere in Webmaster account but cannot find it anywhere. The widget has been removed.
After looking for sometime, it looks like Widget has been removed from Google Webmaster Central. However, it can be found at Google Support. You do not need to sign up/sign in to Webmaster account to get the 404 page code.
