system-helper: Strip polkit errors before sending them on

We shouldn't put the unsightly dbus error wrapping in the
UI. The cient already strips the wrapping from the system
helper call, but it can't strip the second level itself.

Closes: #2399
Approved by: mwleeds

system-helper/flatpak-system-helper.c

@@ -1475,6 +1475,7 @@ flatpak_authorize_method_handler (GDBusInterfaceSkeleton *interface,
                                                           NULL, &error);
       if (result == NULL)
         {
+          g_dbus_error_strip_remote_error (error);
           g_dbus_method_invocation_return_error (invocation, G_DBUS_ERROR, G_DBUS_ERROR_FAILED,
                                                  "Authorization error: %s", error->message);
           return FALSE;