CURA-5035 No more crash without internet connection

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

plugins/Toolbox/resources/qml/Toolbox.qml

@@ -46,6 +46,11 @@ Window
                 id: viewLoading
                 visible: toolbox.viewCategory != "installed" && toolbox.viewPage == "loading"
             }
+            ToolboxErrorPage
+            {
+                id: viewErrored
+                visible: toolbox.viewCategory != "installed" && toolbox.viewPage == "errored"
+            }
             ToolboxDownloadsPage
             {
                 id: viewDownloads

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
+        }
+    }
+}

plugins/Toolbox/src/Toolbox.py

@@ -314,6 +314,7 @@ class Toolbox(QObject, Extension):
         return
 
     def resetDownload(self):
+        if self._download_reply:
             self._download_reply.abort()
             self._download_reply.downloadProgress.disconnect(self._onDownloadProgress)
         self._download_reply = None
@@ -333,11 +334,13 @@ class Toolbox(QObject, Extension):
 
         if reply.error() == QNetworkReply.TimeoutError:
             Logger.log("w", "Got a timeout.")
+            self.setViewPage("errored")
             self.resetDownload()
             return
 
         if reply.error() == QNetworkReply.HostNotFoundError:
             Logger.log("w", "Unable to reach server.")
+            self.setViewPage("errored")
             self.resetDownload()
             return