Implement config service cache on top of datastore

appengine/gaeconfig/cache.go

Index: appengine/gaeconfig/cache.go
diff --git a/appengine/gaeconfig/cache.go b/appengine/gaeconfig/cache.go
index 06743dcdc21300c2bc3f0509bfba372577a9c8e3..5e2fadd088c3e3be66caca6c9154f59163461be7 100644
--- a/appengine/gaeconfig/cache.go
+++ b/appengine/gaeconfig/cache.go
@@ -81,11 +81,7 @@ func memcacheBackend(b config.Backend, exp time.Duration) config.Backend {
 
 			default:
 				// Unknown memcache error.
-				log.Fields{
-					log.ErrorKey: err,
-					"key":        k,
-				}.Warningf(c, "Failed to decode memcached config.")
-				return l(c, key, nil)
+				return nil, errors.Annotate(err).Reason("failed to load memcache config").Err()
 			}
 		},
 	}