webdav: generate error page when template couldn't be found

The webdav door uses StringTemplate to generate HTML pages for
directory listing and error pages, using hard-coded template
names that the corresponding template files must provide.

One reason for using StringTemplate is to allow site customisation
of the output.  However, dCache behaves badly if the hard-coded
template name isn't found; rather than logging a meaningful
error, a NPE is triggered.

This patch provides a hard-coded "broken dCache" page that describes
the problem so that the admin can fix their own site.

Target: master
Patch: http://rb.dcache.org/r/6981/
Acked-by: Tigran Mkrtchyan