[gnome-session] systemd: Add more details to systemd setup error

[Bastien Nocera <hadess src gnome org>]

commit fd122eda5318dc5dd45bf8b77afe9de333bef952
Author: Bastien Nocera <hadess hadess net>
Date:   Tue May 30 12:01:44 2017 +0200

    systemd: Add more details to systemd setup error
    
    By adding the contents of the error reported.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=781173

 gnome-session/gsm-systemd.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/gnome-session/gsm-systemd.c b/gnome-session/gsm-systemd.c
index c64aaa7..ee43bfa 100644
--- a/gnome-session/gsm-systemd.c
+++ b/gnome-session/gsm-systemd.c
@@ -318,10 +318,12 @@ gsm_systemd_init (GsmSystemd *manager)
                                       0,
                                       G_MAXINT,
                                       NULL,
-                                      NULL);
+                                      &error);
         if (res == NULL) {
                 g_warning ("Could not get session id for session. Check that logind is "
-                           "properly installed and pam_systemd is getting used at login.");
+                           "properly installed and pam_systemd is getting used at login: %s",
+                           error->message);
+                g_error_free (error);
                 return;
         }