[GUI] Show error in a dialog when pull or export fails, instead of crashing the whole app

DiscordChatExporter.Gui/Views/Dialogs/MessageBoxView.xaml

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+<UserControl
+    x:Class="DiscordChatExporter.Gui.Views.Dialogs.MessageBoxView"
+    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+    xmlns:dialogs="clr-namespace:DiscordChatExporter.Gui.ViewModels.Dialogs"
+    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+    xmlns:s="https://github.com/canton7/Stylet"
+    MinWidth="500"
+    d:DataContext="{d:DesignInstance Type=dialogs:MessageBoxViewModel}"
+    d:DesignHeight="450"
+    d:DesignWidth="800"
+    Style="{DynamicResource MaterialDesignRoot}"
+    mc:Ignorable="d">
+    <Grid>
+        <Grid.RowDefinitions>
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="*" />
+            <RowDefinition Height="Auto" />
+        </Grid.RowDefinitions>
+
+        <!--  Title  -->
+        <TextBlock
+            Grid.Row="0"
+            Margin="16"
+            FontSize="17"
+            Text="{Binding Title}"
+            TextTrimming="CharacterEllipsis"
+            ToolTip="{Binding Title}" />
+
+        <!--  Message  -->
+        <TextBlock
+            Grid.Row="1"
+            Margin="16,0,16,16"
+            Text="{Binding Message}"
+            TextWrapping="Wrap" />
+
+        <!--  Close  -->
+        <Button
+            Grid.Row="2"
+            Margin="8"
+            HorizontalAlignment="Right"
+            Command="{s:Action Close}"
+            Content="CLOSE"
+            IsCancel="True"
+            IsDefault="True"
+            Style="{DynamicResource MaterialDesignFlatButton}" />
+    </Grid>
+</UserControl>

DiscordChatExporter.Gui/Views/Dialogs/MessageBoxView.xaml.cs

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+namespace DiscordChatExporter.Gui.Views.Dialogs
+{
+    public partial class MessageBoxView
+    {
+        public MessageBoxView()
+        {
+            InitializeComponent();
+        }
+    }
+}