"Not" checker will fail due to error string is not empty

[shinningstar]

https://github.com/go-check/check/blob/v1/helpers.go#L215

Method "internalCheck" does not only check the result returned by checker, but also check if error message is empty. The problem is about the "Not" checker. It only inverts the result, but not clears error message. It does make sense that every checker returns error message if result is false. So internalCheck reports fail when using Not checker. I think it should be a bug here.