webdav: Log StringTemplate errors through slf4j

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6195/
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8179
(cherry picked from commit 93718be864252dcf025cc6cf43e977ba1cbb2b64)