Checkmk Werk 14282: Master control: Fix rendering errors of dead sites

ID: 14282 Title: Master control: Fix rendering errors of dead sites Component: Multisite Level: 1 Class: Bug fix Version: 2.2.0i1 Rendering the master control snapin could fail with an exception like "TypeError: Unsupported type <class 'livestatus.MKLivestatusSocketError'>" in case a site was not reachable.