Add util::Status and util::StatusOr

Cloned from https://github.com/google/lmctfy/tree/master/util/task with a bit less dep fluff.

Hmm, I'm not sure that buys us much other than a confusingly named header under util/ ?
I've popped a comment above the enum mentioning the proto values though, and I've also added a comment at the top saying where we lifted it all from.