In Windows, filesystem redirectors can map WebDAV resources to UNC
paths, or to drive-based "local" paths; so a WebDAV URI of the form
"http://WebDADServer/root/directory/File.ext" may be accessed using
"\\WebDADServer\root\directory\File.ext" or "Z:\directory\File.ext".

When using these paths, failure to create a lockfile aside the opened
document should not be considered an error; so this patch checks for
this.

Regression from commit 6ca3b364.

Change-Id: I1de55b66447dc91d22b6d2b5b121de96bf32e4ee
Reviewed-on: https://gerrit.libreoffice.org/53070Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>