Apache Forrest code update (bug FOR-284)

code update
by
DC
(fixes bug FOR-284)

:

Reinstate error reporting for broken "site:" and "ext:" links
which are due to missing entries in site.xml or typos in site: links.
The text "error:" is also prepended.
This is a workaround for bug FOR-284 "link rewriting broken
when linking to xml source views which contain site: links".
This latter issue is handled by the core. However, if
your project uses a Cocoon
cli.xconf
then you will need add the same "exclude" entries.