Tracking issue for StatusOr Experimental API

**Allow null?**
StatusOr needs to be as expressive as a method return value, although it will generally not be null. Can potentially use StatusOr<@NonNull> in cases it should be disallowed.

**Should getValue() throw if there is no value?**
Go with just getValue() throwing IllegalStateException for now. Can add getValueOrNull, getValueOrDefault, getValueOrThrow as we find usecases. Seems the majority case should check getStatus() first.