fetch: support v2 w/o manifest on old WWW

fetch: support v2 w/o manifest on old WWW

There may still be pre-manifest.js.gz versions of
PublicInbox::WWW running and serving v2 inboxes.

While -clone and "add-external --mirror" were working, -fetch
was failing due to 301 redirect to $INBOX_URL/manifest.js.gz/
and not the expected 404.  Update the code to deal with a JSON
decode error (from the 301) and ensure v2 epochs detection is
correct (and not using a shadowed variable).