[glib/portal] Set an error on failure

[Matthias Clasen <matthiasc src gnome org>]

commit 9d59e45961f11393093fed39cd62df56c3ba18fc
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Jul 6 19:29:13 2016 -0400

    Set an error on failure
    
    'has no owner' seems close enough, that is what we are seeing
    here.

 gio/gnetworkmonitorportal.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/gio/gnetworkmonitorportal.c b/gio/gnetworkmonitorportal.c
index 438fcf5..0d3aeb1 100644
--- a/gio/gnetworkmonitorportal.c
+++ b/gio/gnetworkmonitorportal.c
@@ -135,6 +135,10 @@ g_network_monitor_portal_initable_init (GInitable     *initable,
   if (!name_owner)
     {
       g_object_unref (proxy);
+      g_set_error (error,
+                   G_DBUS_ERROR,
+                   G_DBUS_ERROR_NAME_HAS_NO_OWNER,
+                   "Desktop portal not found");
       return FALSE;
     }