Add an error message to download dialog if models.json can't be retrieved.

gpt4all-chat/qml/ModelDownloaderDialog.qml

@@ -55,6 +55,13 @@ Dialog {
             color: theme.textColor
         }
 
+        Label {
+            visible: !Download.modelList.length
+            anchors.centerIn: parent
+            text: qsTr("Network error: could not retrieve http://gpt4all.io/models/models.json")
+            color: theme.mutedTextColor
+        }
+
         ScrollView {
             id: scrollView
             ScrollBar.vertical.policy: ScrollBar.AlwaysOn