DavDocumentsProvider: Log a warning for responses of type OTHER. (#400)

If a WebDAV server is misconfigured (for example, behind an HTTP proxy that
strips a URL prefix and doesn't correctly forward this to the WebDAV server)
then PROPFIND can return resources whose URLs don't match the request URL. These
are resolved by dav4jvm as HrefRelation.OTHER.

Currently this situation produces no output at all in DAVx5 (logs or app) and
the WebDAV share appears accessible but empty. It is possible to create files in
the share, but not to see them again afterwards!

Of course a misconfigured server isn't the WebDAV client's responsibility to
resolve, but adding a warning in the log provides an extra clue for anyone
trying to debug it.