Improve error message

LeStorage/StoredSingleton.swift

@@ -23,4 +23,14 @@ public class StoredSingleton<T: Storable>: StoredCollection<T> {
         return self.items.first
     }
     
+    // MARK: - Protects from use
+    
+//    public override func addOrUpdate(instance: T) throws {
+//        fatalError("method unavailable for StoredSingleton")
+//    }
+    
+    public override func addOrUpdate(contentOfs sequence: any Sequence<T>) throws {
+        fatalError("method unavailable for StoredSingleton, use update")
+    }
+    
 }