skip unkown cluster ids when opening the clusters

if someone opens a cluster manually it doesn't exits anymore in the cluster.
However the clusterIndex has still the same size.
Which makes the openClusters function fail.
This an easy workaround to get around this problem.

Server/Server/home.aspx

@@ -92,7 +92,12 @@ Please be patient and don't refresh the page.</p>
     function openClusters() {
         var newClusters = [];
         for (var i = 0; i < clusters.length; i++) {
+            try {
                 network.openCluster(clusters[i].id);
+            }
+            catch {
+                continue;
+            }
         }
         if (document.getElementById('stabilizeCheckbox').checked === true) {
             network.stabilize();