Merge request: http://users.puzzling.org/users/mary/bzr/planet/branches/original-url-tracking/

[Mary Gardiner]

This branch is a much smaller change than my last. The motivation for
this branch is that, with ERROR level logging, I've been getting a lot
of messages along these lines:

ERROR:planet:Error 404 while updating feed <http://www.blogger.com/maintenance.html>

I don't have the feed http://www.blogger.com/maintenance.html in my ini
file and never did quite manage to find where it was being redirected
from.

Error messages of this kind, only where the feed has been redirected,
will now read like this:

ERROR:planet:Error 404 while updating feed <http://www.blogger.com/maintenance.html> (formerly <http://bastubis.blogspot.com/atom.xml>)

-Mary