Useful message on empty logfiles

[GoogleCodeExporter]

On the current Buildbot master installation, logs of elderly builds are
purged whereas the links to the build page (e.g.
http://termite.go-oo.org/buildbot/builders/Fedora-10-i386/builds/269)
remains valid but shows an empty page.
It would be more useful to show an informative message, e.g. "The logs of
this build has been purged".

Original issue reported on code.google.com by [email protected] on 12 May 2009 at 2:23