Add new IndexNotFound exception (#437)

Add new IndexNotFound exception to be used by memory dbs and clients

service/Abstractions/MemoryStorage/IndexNotFound.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+
+namespace Microsoft.KernelMemory.MemoryStorage;
+
+public class IndexNotFound : KernelMemoryException
+{
+    /// <inheritdoc />
+    public IndexNotFound() { }
+
+    /// <inheritdoc />
+    public IndexNotFound(string message) : base(message) { }
+
+    /// <inheritdoc />
+    public IndexNotFound(string message, Exception? innerException) : base(message, innerException) { }
+}