status picks the icon. success, error, info and warning draw an icon from your icon pack in the status color. 403, 404 and 500 draw a built-in illustration. The default is info.
Pick by what happened: 404 for an address that leads nowhere, 403 for a signed-in reader who is not allowed, 500 for a failure on your side. An expired session is not a 403. Send the reader to sign in, so they keep what they were doing.