CURA-5035 No more crash without internet connection

Still needs a timer or something to show the fetch failed though.

plugins/Toolbox/resources/qml/ToolboxErrorPage.qml

@@ -0,0 +1,22 @@
+// Copyright (c) 2018 Ultimaker B.V.
+// Toolbox is released under the terms of the LGPLv3 or higher.
+
+import QtQuick 2.7
+import QtQuick.Controls 1.4
+import QtQuick.Controls.Styles 1.4
+
+Rectangle
+{
+    id: page
+    width: parent.width
+    height: parent.height
+    color: "transparent"
+    Label
+    {
+        text: catalog.i18nc("@info", "Could not connect to the Cura Package database. Please check your connection.")
+        anchors
+        {
+            centerIn: parent
+        }
+    }
+}