They touched on this in their SRE post last week: https://cloudplatform.googleblog.com/2017/03/reliable-releas...
I like that -- reminds me of aviation, where a go-around is normal. If your approach to landing isn't stabilized, you're too high, too low, too fast, too slow, etc. don't try to save it. Go around and try again.
Next we plan to automate the process - 1 in 10 rollouts will actually be a rollout, a rollback, and another rollout, checking system health at each step.