Fixes an issue in Session.LoadDataTypeSystem. (#2166)

Starting with line 1857 a dictionary of namespaces is populated in an defensive manner. If a namespace value can't be read, it is silently ignored. If this happens, it will result in an KeyNotFoundException when populating the imports dictionary.
This fix adds a warning if a namespace value can't be read and populates the imports dictionary also defensivly.

Libraries/Opc.Ua.Client/Session.cs

@@ -1860,16 +1860,20 @@ public async Task<DataDictionary> LoadDataDictionary(ReferenceDescription dictio
                 {
                     namespaces[((NodeId)referenceNodeIds[ii])] = (string)nameSpaceValues[ii];
                 }
+                else
+                {
+                    Utils.LogWarning("Failed to load namespace {0}: {1}", namespaceNodeIds[ii], errors[ii]);
+                }
             }
 
             // build the namespace/schema import dictionary
             var imports = new Dictionary<string, byte[]>();
             foreach (var r in references)
             {
                 NodeId nodeId = ExpandedNodeId.ToNodeId(r.NodeId, NamespaceUris);
-                if (schemas.TryGetValue(nodeId, out var schema))
+                if (schemas.TryGetValue(nodeId, out var schema) && namespaces.TryGetValue(nodeId, out var ns))
                 {
-                    imports[namespaces[nodeId]] = schema;
+                    imports[ns] = schema;
                 }
             }