add txn uncertainty test & fix a bug

Added missing Txn Uncertainty test and along the way realized that forgetting to add ReadWithinUncertaintyIntervalError to GoError() is not a great thing to do as the error simply evaporates.
I think we should add a method to proto/data.go to extend proto.Timestamp with this functionality. We've had need of it elsewhere in the actual code.

    func (t Timestamp) Add(walltime int64, logical int32) Timestamp {
      return Timestamp{
        WallTime: t.WallTime + wallTime,
        Logical: t.Logical + logical,
      }
    }