[#56] Windows: Fail gracefully while saving

Elwig/Windows/ChartWindow.xaml.cs

@@ -661,6 +661,9 @@ namespace Elwig.Windows {
         }
 
         private async void SaveButton_Click(object sender, RoutedEventArgs e) {
+            SaveButton.IsEnabled = false;
+            Mouse.OverrideCursor = Cursors.AppStarting;
+
             try {
                 using (var ctx = new AppDbContext()) {
                     var origData = BillingData.FromJson(PaymentVar.Data);
@@ -677,7 +680,12 @@ namespace Elwig.Windows {
                 var str = "Der Eintrag konnte nicht in der Datenbank gespeichert werden!\n\n" + exc.Message;
                 if (exc.InnerException != null) str += "\n\n" + exc.InnerException.Message;
                 MessageBox.Show(str, "Auszahlungsvariante speichern", MessageBoxButton.OK, MessageBoxImage.Error);
+                SaveButton.IsEnabled = true;
+                return;
+            } finally {
+                Mouse.OverrideCursor = null;
             }
+
             LockContext = true;
             SetHasChanged(false);
         }