Clearly define error happened while filltering Ambassador hosts by an…

…notation

As suggested by alebedev87 in kubernetes-sigs#2633 (comment)

Co-authored-by: Andrey Lebedev <alebedev87@gmail.com>

source/ambassador_host.go

@@ -137,7 +137,7 @@ func (sc *ambassadorHostSource) Endpoints(ctx context.Context) ([]*endpoint.Endp
 	// Filter Ambassador Hosts
 	ambassadorHosts, err = sc.filterByAnnotations(ambassadorHosts)
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to filter Ambassador Hosts")
+		return nil, errors.Wrap(err, "failed to filter Ambassador Hosts by annotation")
 	}
 
 	endpoints := []*endpoint.Endpoint{}