[errors] de-specialize Transient in favor of Tags.

common/config/validation/validation_test.go

Index: common/config/validation/validation_test.go
diff --git a/common/config/validation/validation_test.go b/common/config/validation/validation_test.go
index ebf7c0a81479f65d389765a78034fad462f347fc..c410c3ed122c2969b3566789146215a73365711e 100644
--- a/common/config/validation/validation_test.go
+++ b/common/config/validation/validation_test.go
@@ -7,7 +7,6 @@ package validation
 import (
 	"testing"
 
-	"github.com/luci/luci-go/common/errors"
 	"github.com/luci/luci-go/common/logging"
 
 	. "github.com/smartystreets/goconvey/convey"
@@ -40,8 +39,12 @@ func TestValidation(t *testing.T) {
 
 		singleErr := err.(*Error).Errors[0]
 		So(singleErr.Error(), ShouldEqual, `in "file.cfg" (ctx 123): blah zzz`)
-		So(errors.ExtractData(singleErr, "file"), ShouldEqual, "file.cfg")
-		So(errors.ExtractData(singleErr, "element"), ShouldResemble, []string{"ctx 123"})
+		d, ok := fileTag.ValueIn(singleErr)
+		So(ok, ShouldBeTrue)
+		So(d.(string), ShouldEqual, "file.cfg")
+		d, ok = element0Tag.ValueIn(singleErr)
+		So(ok, ShouldBeTrue)
+		So(d.(string), ShouldResemble, "ctx 123")
 	})
 
 	Convey("Regular usage", t, func() {