The aggregate plugin's handling of http 401 (moved permanently) could be
improved. Per [[rfc 1945]]:

> The requested resource has been assigned a new permanent URL
> and any future references to this resource should be done
> using that URL.

So ideally aggregate would notice the 401 and use the new url henceforth.

It's a little tricky because the aggregate plugin can't just edit the page and
change the url in the preprocessor directive. (Because committing such an
edit would be .. hard.) Also, aggregate directives may also include a separate
url for the site, which may also have changed. Perhaps the thing to do is
record the new url in the aggregate plugin's state file, and change the message
to "Processed ok (new url http://..)", and let the user deal with updating
the page later.