Say why the folders cannot be made instead of dying

Something already sitting where Documents/N1MM Logger+ should go — a file, or
a link to somewhere that is not there — threw out of Directory.CreateDirectory
before the program had a window, so the operator got a stack trace and nothing
else.

CreateFolders answers with what stopped it, and the program shows it in a
window naming the path and stops there. NONEMM_HOME moves the whole lot
somewhere else, for a station that keeps its files off the Documents folder.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RoGtneMQaz4M9w7Kk49AVD

src/Nonemm.App/Windows/StartupProblemWindow.cs

@@ -0,0 +1,52 @@
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Media;
+
+namespace Nonemm.App.Windows;
+
+/// What the operator sees when the program cannot make the folders it keeps
+/// everything in. There is no log, no settings and no entry window at that
+/// point, so this says what is wrong and what to do about it, and closing it
+/// ends the program.
+public sealed class StartupProblemWindow : Window
+{
+    public StartupProblemWindow(string problem, string root)
+    {
+        Title = "Nonemm cannot start";
+        Width = 560;
+        SizeToContent = SizeToContent.Height;
+        WindowStartupLocation = WindowStartupLocation.CenterScreen;
+        Content = new StackPanel
+        {
+            Margin = new Avalonia.Thickness(18),
+            Spacing = 10,
+            Children =
+            {
+                new TextBlock
+                {
+                    Text = "The program keeps its files with N1MM's, and could not make them:",
+                    TextWrapping = TextWrapping.Wrap,
+                },
+                new TextBlock { Text = problem, TextWrapping = TextWrapping.Wrap, FontWeight = FontWeight.Bold },
+                new TextBlock
+                {
+                    Text = $"The folder it works in is {root}. Setting NONEMM_HOME to another "
+                        + "path moves the whole lot somewhere else.",
+                    TextWrapping = TextWrapping.Wrap,
+                    FontSize = 11,
+                    Opacity = 0.75,
+                },
+                new Button
+                {
+                    Content = "Quit",
+                    HorizontalAlignment = HorizontalAlignment.Right,
+                    IsDefault = true,
+                },
+            },
+        };
+        if (((StackPanel)Content).Children[^1] is Button quit)
+        {
+            quit.Click += (_, _) => Close();
+        }
+    }
+}