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/App.axaml.cs

@@ -17,7 +17,12 @@ public partial class App : Application
         if (ApplicationLifetime is IClassicDesktopStyleApplicationLifetime desktop)
         {
             UserPaths paths = UserPaths.Default();
-            paths.CreateFolders();
+            if (paths.CreateFolders() is { } problem)
+            {
+                desktop.MainWindow = new StartupProblemWindow(problem, paths.Root);
+                base.OnFrameworkInitializationCompleted();
+                return;
+            }
             Settings settings = Settings.Load(paths.SettingsFile);
             Themes.Use(settings.Theme);
             Themes.Changed += (_, _) => ApplyTheme();