thunderbolt: specific message for unknown security

If bolt cannot determine the security level, which in theory should
never happen[1] the standard no-thunderbolt message seems not
entirely appropriate. Instead show a different message that is more
tailored to this unusual case.

[1] The following (unlikely) reasons would invoke such a scenario:
   - new security level not supported by boltd
   - old hardware with not force-power support and the thunderbolt
     controller has not yet been powered up

panels/thunderbolt/cc-bolt-panel.c

@@ -505,7 +505,7 @@ cc_bolt_panel_name_owner_changed (CcBoltPanel *panel)
       break;
 
     case BOLT_SECURITY_UNKNOWN:
-      text = NULL;
+      text = _("Thunderbolt security level could not be determined.");;
       break;
     }