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

tests/Nonemm.App.Tests/UserPathsTests.cs

@@ -20,7 +20,7 @@ public class UserPathsTests : IDisposable
     {
         UserPaths paths = new(root);
 
-        paths.CreateFolders();
+        Assert.Null(paths.CreateFolders());
 
         string[] made = [.. Directory.GetDirectories(root).Select(Path.GetFileName).OfType<string>().Order()];
         Assert.Equal(
@@ -62,4 +62,32 @@ public class UserPathsTests : IDisposable
 
         Assert.Equal(Path.Combine(root, "Nonemm", "settings.json"), paths.SettingsFile);
     }
+
+    /// A file where the folder should be stops the program starting, so it says
+    /// so rather than throwing at an operator who has no window to read it in.
+    [Fact]
+    public void AFileWhereTheFolderShouldBeIsReported()
+    {
+        Directory.CreateDirectory(root);
+        string inTheWay = Path.Combine(root, "N1MM Logger+");
+        File.WriteAllText(inTheWay, "not a folder");
+        UserPaths paths = new(inTheWay);
+
+        string? problem = paths.CreateFolders();
+
+        Assert.NotNull(problem);
+        Assert.Contains(inTheWay, problem);
+        Assert.Contains("is a file", problem);
+    }
+
+    [Fact]
+    public void FoldersThatAreAlreadyThereAreLeftAlone()
+    {
+        UserPaths paths = new(root);
+        paths.CreateFolders();
+        File.WriteAllText(Path.Combine(paths.Databases, "om5m.s3db"), "log");
+
+        Assert.Null(paths.CreateFolders());
+        Assert.Equal("log", File.ReadAllText(Path.Combine(paths.Databases, "om5m.s3db")));
+    }
 }