feat: define NotFoundOrForbidden as common error type

Author: bitfl0wer

polyproto/main.tsp

@@ -3,3 +3,16 @@ import "./core/main.tsp";
 import "./mls/main.tsp";
 
 namespace polyproto;
+
+using Http;
+
+namespace models.errors {
+    @doc("""
+        The client either does not have the access rights to access this resource, or this resource
+        does not exist. The ambiguity about the actual error has the purpose of protecting the targeted
+        resource from information leaks.
+        """)
+    model NotFoundOrForbidden {
+        ...NotFoundResponse;
+    }
+}