add Unwrap() method of yoError

[qua-iy]

Spanner will retry ReadWriteTransaction when transaction was aborted, by checking spanner.Error using Unwrap method. Since yo wraps spanner.Error by yoError, yoError should to support Unwrap() method.

[kazegusuri]

I think it's okay to add Unwrap() by default. However the Unwrap is not used inside spanner library.
Spanner library uses errors.As() or status.FromError() to know gRPC status code. yoError does not support As(), so returns the original status code by status.FromError(). That's why yoError implements GRPCStatus().

[kazegusuri]

ah it's my misunderstanding. Unwrap() is called inside errors.As(). LGTM.