Improve presentation of "Update Issues" error message

Assemble and format the string in one place rather than two, and add
more line breaks between the summary and the full text.

discover/qml/UpdatesPage.qml

@@ -18,7 +18,7 @@ DiscoverPage
     readonly property var resourcesUpdatesModel: ResourcesUpdatesModel {
         id: resourcesUpdatesModel
         onPassiveMessage: {
-            desc.text = xi18nc("@info", "%1<nl/><nl/>Please report this issue to the packagers of your distribution.", message);
+            desc.text = message;
             sheet.sheetOpen = true;
         }
         onIsProgressingChanged: {

libdiscover/backends/PackageKitBackend/PackageKitUpdater.cpp

@@ -500,7 +500,11 @@ void PackageKitUpdater::errorFound(PackageKit::Transaction::Error err, const QSt
 {
     if (err == PackageKit::Transaction::ErrorNoLicenseAgreement)
         return;
-    Q_EMIT passiveMessage(QStringLiteral("%1\n%2").arg(PackageKitMessages::errorMessage(err), error));
+    QString finalMessage = xi18nc("@info",
+                                  "%1:<nl/><nl/>%2<nl/><nl/>Please report this issue to the packagers of your distribution.",
+                                  PackageKitMessages::errorMessage(err),
+                                  error);
+    Q_EMIT passiveMessage(finalMessage);
     qWarning() << "Error happened" << err << error;
 }