Code review comment for lp:~mbp/bzr/704195-plugin-warnings

I'll update a couple of things before landing based on conversations
with Jelmer:

 * the code that formats the list could fold both the loaded and
failed-to-load plugins together, and be a bit simpler
 * should move into the 2.4 NEWS

Note that with this change, missing-dependency plugins will continue
to not load; we just represent that differently.

--
Martin