Add some error handling

DiscordChatExporter/Views/ErrorDialog.ammy

@@ -0,0 +1,25 @@
+using MaterialDesignThemes.Wpf
+
+UserControl "DiscordChatExporter.Views.ErrorDialog" {
+  DataContext: bind ErrorViewModel from $resource Container
+    Width: 250
+
+    StackPanel {
+      // Message
+      TextBlock {
+        Margin: 8
+        FontSize: 16
+        HorizontalAlignment: Center
+        TextWrapping: WrapWithOverflow
+        Text: bind Message
+      }
+
+      // OK
+      Button {
+        Command: DialogHost.CloseDialogCommand
+        Content: "OK"
+        Margin: 8
+        Style: resource dyn "MaterialDesignFlatButton"
+      }
+    }
+}

DiscordChatExporter/Views/ErrorDialog.ammy.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace DiscordChatExporter.Views
+{
+    public partial class ErrorDialog
+    {
+        public ErrorDialog()
+        {
+            InitializeComponent();
+        }
+    }
+}

DiscordChatExporter/Views/MainWindow.ammy.cs

@@ -17,6 +17,7 @@ namespace DiscordChatExporter.Views
             InitializeComponent();
             Title += $" v{Assembly.GetExecutingAssembly().GetName().Version}";
 
+            Messenger.Default.Register<ShowErrorMessage>(this, m => DialogHost.Show(new ErrorDialog()).Forget());
             Messenger.Default.Register<ShowSettingsMessage>(this, m => DialogHost.Show(new SettingsDialog()).Forget());
         }