Improve error message.

Also fix incorrect DECREF.

Author: ZeroIntensity

Objects/genericaliasobject.c

@@ -529,8 +529,10 @@ _Py_subs_parameters(PyObject *self, PyObject *args, PyObject *parameters, PyObje
                 Py_DECREF(newargs);
                 Py_DECREF(item);
                 Py_XDECREF(tuple_args);
+                PyObject *original = PyTuple_GET_ITEM(args, iarg);
                 PyErr_Format(PyExc_TypeError,
-                             "expected a tuple, not %T", arg);
+                             "expected __typing_subst__ of %T objects to return a tuple, not %T",
+                             original, arg);
                 Py_DECREF(arg);
                 return NULL;
             }
@@ -540,7 +542,7 @@ _Py_subs_parameters(PyObject *self, PyObject *args, PyObject *parameters, PyObje
             if (jarg < 0) {
                 Py_DECREF(item);
                 Py_XDECREF(tuple_args);
-                Py_DECREF(newargs);
+                /* newargs was stolen */
                 return NULL;
             }
         }