With the adoption of Go module, GOPATH is not need any more
But github.com/etcd-io/gofail doesn't support packages outside the GOPATH
To overcome the drawback, this commit replace it with our forked one
We import "github.com/pkg/errors" in the code, and actually put
the "github.com/pingcap/errors" there in the vendor path.
That's a dirty hack, and prevent the introduce of Go module.
