Milo: Use custom config caching layer

luci_config/server/cfgclient/backend/erroring/erroring.go

Index: luci_config/server/cfgclient/backend/erroring/erroring.go
diff --git a/luci_config/server/cfgclient/backend/erroring/erroring.go b/luci_config/server/cfgclient/backend/erroring/erroring.go
index cbd8ef89d124c27dcb4d2355b1708974cc969856..64a1079eed13e35fb82f8958bf0043f593a4c778 100644
--- a/luci_config/server/cfgclient/backend/erroring/erroring.go
+++ b/luci_config/server/cfgclient/backend/erroring/erroring.go
@@ -9,8 +9,11 @@
 package erroring
 
 import (
+	"errors"
 	"net/url"
 
+	"github.com/luci/luci-go/common/config"
+	"github.com/luci/luci-go/common/config/impl/memory"
 	"github.com/luci/luci-go/luci_config/server/cfgclient/backend"
 
 	"golang.org/x/net/context"
@@ -49,3 +52,10 @@ func (e erroringImpl) GetAll(c context.Context, t backend.GetAllTarget, path str
 func (e erroringImpl) ConfigSetURL(c context.Context, configSet string, p backend.Params) (url.URL, error) {
 	return url.URL{}, e.err
 }
+
+func (e erroringImpl) GetConfigInterface(c context.Context, a backend.Authority) config.Interface {
+	emptySet := map[string]memory.ConfigSet{}
+	i := memory.New(emptySet)
+	memory.SetError(i, errors.New("error"))
+	return i
+}