I noticed logs on one of my machines where it can't auto-update with
scary log spam about "failed to apply tailnet-wide default for
auto-updates".
This avoids trying to do the EditPrefs if we know it's just going to
fail anyway.
Updates #282
Change-Id: Ib7db3b122185faa70efe08b60ebd05a6094eed8c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>