"not subscriptable" should be a bit more understandable than "unsubscriptable".

Objects/abstract.c

@@ -153,7 +153,7 @@ PyObject_GetItem(PyObject *o, PyObject *key)
 					  "be integer, not '%.200s'", key);
 	}
 
-	return type_error("'%.200s' object is unsubscriptable", o);
+	return type_error("'%.200s' object is not subscriptable", o);
 }
 
 int