Improve error on router without service.

Co-authored-by: Mathieu Lonjaret <mathieu.lonjaret@gmail.com>

pkg/server/router/router.go

@@ -2,6 +2,7 @@ package router
 
 import (
 	"context"
+	"errors"
 	"net/http"
 
 	"github.com/containous/alice"
@@ -148,6 +149,10 @@ func (m *Manager) buildHTTPHandler(ctx context.Context, router *runtime.RouterIn
 	}
 	rm := m.modifierBuilder.Build(ctx, qualifiedNames)
 
+	if router.Service == "" {
+		return nil, errors.New("the service is missing on the router")
+	}
+
 	sHandler, err := m.serviceManager.BuildHTTP(ctx, router.Service, rm)
 	if err != nil {
 		return nil, err