main: Lower fallback warning when running in GDM

It is currently expected for the fallback to happen. So hide the errors,
but still log a single message to inform users about it.

gnome-session/main.c

@@ -543,7 +543,7 @@ main (int argc, char **argv)
 
 #ifdef HAVE_SYSTEMD
         gsm_util_export_user_environment (&error);
-        if (error) {
+        if (error && !g_getenv ("RUNNING_UNDER_GDM")) {
                 g_warning ("Failed to upload environment to systemd: %s", error->message);
                 g_clear_error (&error);
         }
@@ -567,7 +567,7 @@ main (int argc, char **argv)
                  * in a previous session
                  */
                 gsm_util_systemd_reset_failed (&error);
-                if (error) {
+                if (error && !g_getenv ("RUNNING_UNDER_GDM")) {
                         g_warning ("Failed to reset failed state of units: %s", error->message);
                         g_clear_error (&error);
                 }
@@ -581,6 +581,9 @@ main (int argc, char **argv)
                 }
 
                 /* We could not start the unit, fall back. */
+                if (g_getenv ("RUNNING_UNDER_GDM"))
+                        g_message ("Falling back to non-systemd startup procedure. This is expected to happen for GDM sessions.");
+                else
                         g_warning ("Falling back to non-systemd startup procedure due to error: %s", error->message);
                 g_clear_error (&error);
         }