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

tumble/service.go

Index: tumble/service.go
diff --git a/tumble/service.go b/tumble/service.go
index 187351ff8cc5841ec269b2674c7b9173fdd91157..8acdb4e77edc5db2b40fc70376562eb94a09600e 100644
--- a/tumble/service.go
+++ b/tumble/service.go
@@ -15,6 +15,7 @@ import (
 	"github.com/luci/luci-go/appengine/gaemiddleware"
 	"github.com/luci/luci-go/common/errors"
 	"github.com/luci/luci-go/common/logging"
+	"github.com/luci/luci-go/common/retry/transient"
 	"github.com/luci/luci-go/common/sync/parallel"
 	"github.com/luci/luci-go/server/router"
 
@@ -225,7 +226,7 @@ func (s *Service) ProcessShardHandler(ctx *router.Context, loop bool) {
 	if err != nil {
 		logging.Errorf(c, "failure! %s", err)
 
-		if errors.IsTransient(err) {
+		if transient.Tag.In(err) {
 			rw.Header().Add(transientHTTPHeader, "true")
 		}
 		rw.WriteHeader(http.StatusInternalServerError)