kitchen: fix bootstrap status
kitchen: fix bootstrap status

Mark bootstrap step exception if bootstrapping failed

Context: https://luci-milo.appspot.com/swarming/prod/2f9d53d578523310

R=estaab@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2107733005

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go