vendor: Update vendor logic

vendor/github.com/pkg/errors/cause.go

@@ -0,0 +1,29 @@
+// +build !go1.13
+
+package errors
+
+// Cause recursively unwraps an error chain and returns the underlying cause of
+// the error, if possible. An error value has a cause if it implements the
+// following interface:
+//
+//     type causer interface {
+//            Cause() error
+//     }
+//
+// If the error does not implement Cause, the original error will
+// be returned. If the error is nil, nil will be returned without further
+// investigation.
+func Cause(err error) error {
+	type causer interface {
+		Cause() error
+	}
+
+	for err != nil {
+		cause, ok := err.(causer)
+		if !ok {
+			break
+		}
+		err = cause.Cause()
+	}
+	return err
+}