Pass on 404

[grahamc]

If any of these calls 404, exit the build with a success immediately:

https://github.com/grahamc/packet-nix-builder/blob/master/drain.sh#L30-L51

https://github.com/grahamc/packet-nix-builder/blob/master/reboot.sh#L29-L35

https://github.com/grahamc/packet-nix-builder/blob/master/restore.sh#L30-L50

A 404 means the spot instance was taken back, not a problem. Probably fine to check for a 404 before running and aborting early, instead of modifying each call. A minor race, but low stakes.