Misleading error message on compose 404

In the method ComposeObject in gslib/gcs_json_api.py, a 404 triggers an exception which sets object_name to dst_obj_name unconditionally, even when the source of the 404 is one of the source objects.

Reproduce with gsutil 4.7:
$ gsutil compose gs://B/F/composite gs://B/F/nonexisting gs://B/F/composite
Composing gs://B/F/composite from 2 component objects.
NotFoundException: 404 gs://B/F/composite does not exist.