webdav: Return HTTP 404 for PROPFIND/PROPPATCH requests on an inexistent
webdav.Dir resource.

Change-Id: If28dc1853cc3f393a53bf8f0e0f0bb87328c3d29
Reviewed-on: https://go-review.googlesource.com/10394
Reviewed-by: Nigel Tao <nigeltao@golang.org>