Clear message instead of crash when data/ or the save is not readable

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

GameServer.cs

@@ -73,19 +73,30 @@ public sealed class GameServer
 
     public bool LoadSave()
     {
-        saveFiles = new List<KeyValuePair<string, byte[]>>();
-        Directory.CreateDirectory(savePath);
-        foreach (string name in SaveFiles)
+        // Erst alles lesen, dann übernehmen: schlägt "reload-save" fehl, bleibt der bisherige Spielstand aktiv
+        var loaded = new List<KeyValuePair<string, byte[]>>();
+        try
         {
-            string file = Path.Combine(savePath, name);
-            if (File.Exists(file)) saveFiles.Add(new(name, File.ReadAllBytes(file)));
+            Directory.CreateDirectory(savePath);
+            foreach (string name in SaveFiles)
+            {
+                string file = Path.Combine(savePath, name);
+                if (File.Exists(file)) loaded.Add(new(name, File.ReadAllBytes(file)));
+            }
         }
-        if (saveFiles.All(f => f.Key != "defaultES2File.txt"))
+        catch (Exception e) when (e is UnauthorizedAccessException or IOException)
+        {
+            Log("Cannot read the save game: " + e.Message);
+            Log(PermissionHint());
+            return false;
+        }
+        if (loaded.All(f => f.Key != "defaultES2File.txt"))
         {
             Log("No save game found in " + savePath);
             Log("Copy your My Summer Car save files there, or start the server with --import-save.");
             return false;
         }
+        saveFiles = loaded;
         Log("Loaded save game (" + saveFiles.Count + " files, " + saveFiles.Sum(f => f.Value.Length) / 1024 + " KB)");
         return true;
     }
@@ -507,6 +518,25 @@ public sealed class GameServer
         }
     }
 
+    /// <summary>
+    /// Hinweis bei fehlenden Dateirechten (Linux/Docker: Dateien gehören einem anderen Benutzer,
+    /// z.B. root oder per SFTP mit anderem Konto hochgeladen).
+    /// </summary>
+    public static string PermissionHint()
+    {
+        string uid = "?";
+        try
+        {
+            string line = File.ReadLines("/proc/self/status").FirstOrDefault(l => l.StartsWith("Uid:"));
+            if (line != null) uid = line.Split(new[] { '\t', ' ' }, StringSplitOptions.RemoveEmptyEntries)[1];
+        }
+        catch { } // nicht Linux
+        return uid == "?"
+            ? "Check the file permissions of the data folder."
+            : $"The server runs as user id {uid}. Fix on the host:  sudo chown -R {uid}:{uid} data   " +
+              "(or set MSCO_UID / MSCO_GID in .env to your own id: `id -u`) - see README \"Permission denied\".";
+    }
+
     void SaveWorld()
     {
         try