buffet: Abort limbo commands. BUG=chromium:420573 TEST=cros_workon_make buffet --test&&manual Change-Id: I2d887cfc70758a1fca0eb1bba96bc42c7c68f88d Reviewed-on: https://chromium-review.googlesource.com/226550 Reviewed-by: Anton Muhin <antonm@chromium.org> Commit-Queue: Anton Muhin <antonm@chromium.org> Tested-by: Anton Muhin <antonm@chromium.org>