[gnome-software] trivial: Fix a warning if gnome-shell is not installed

[Richard Hughes <rhughes src gnome org>]

commit 241685d65cc33380cf0b080fe656e7e20a5df0fc
Author: Richard Hughes <richard hughsie com>
Date:   Fri Nov 4 13:51:56 2016 +0000

    trivial: Fix a warning if gnome-shell is not installed

 src/plugins/gs-plugin-shell-extensions.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/plugins/gs-plugin-shell-extensions.c b/src/plugins/gs-plugin-shell-extensions.c
index 9195516..927cb2e 100644
--- a/src/plugins/gs-plugin-shell-extensions.c
+++ b/src/plugins/gs-plugin-shell-extensions.c
@@ -313,6 +313,7 @@ gs_plugin_add_installed (GsPlugin *plugin,
                                         cancellable,
                                         error);
        if (retval == NULL) {
+               gs_utils_error_convert_gdbus (error);
                gs_utils_error_convert_gio (error);
                return FALSE;
        }