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/Configuration/UserPaths.cs

@@ -33,7 +33,13 @@ public sealed class UserPaths
         Own = Path.Combine(root, "Nonemm");
     }
 
-    public static UserPaths Default() => new(DefaultRoot());
+    /// `NONEMM_HOME` moves the whole lot somewhere else, for a station that
+    /// keeps its files off the Documents folder or runs two copies side by
+    /// side.
+    public static UserPaths Default() =>
+        new(Environment.GetEnvironmentVariable("NONEMM_HOME") is { Length: > 0 } wanted
+            ? wanted
+            : DefaultRoot());
 
     public string Root { get; }
 
@@ -97,12 +103,34 @@ public sealed class UserPaths
             ? Wav
             : Path.Combine(Wav, operatorCallsign.Trim().ToUpperInvariant());
 
-    public void CreateFolders()
+    /// Makes every folder, and answers with what stopped it rather than
+    /// throwing: this runs before there is a window to report anything in, and
+    /// a program that dies with a stack trace tells the operator nothing.
+    public string? CreateFolders()
     {
         foreach (string folder in All())
         {
-            Directory.CreateDirectory(folder);
+            if (Directory.Exists(folder))
+            {
+                continue;
+            }
+            if (File.Exists(folder))
+            {
+                return $"{folder} is a file, not a folder. Move it out of the way and start again.";
+            }
+            try
+            {
+                Directory.CreateDirectory(folder);
+            }
+            catch (Exception failure) when (failure is IOException or UnauthorizedAccessException)
+            {
+                // something is already there that is neither a file nor a
+                // folder — a link to somewhere that is not there is the usual
+                // one — or the disk will not have it
+                return $"{folder} could not be made: {failure.Message}";
+            }
         }
+        return null;
     }
 
     /// Every folder that is made at startup, the root first.