Fix Dir.OpenFile to not return a nil *os.File (a concrete type) when it
should return a nil webdav.File (an interface type).

Fix memFS.RemoveAll to return nil error instead of os.ErrNotExist when
removing a non-existant file, matching os.RemoveAll's documented
semantics.

Change-Id: I84dfb404aca30e084cd46d6bdc94655aab718bc0
Reviewed-on: https://go-review.googlesource.com/2932Reviewed-by: Nigel Tao <nigeltao@golang.org>