don’t report errors for 204 http status codes

204 in webdav context is generally defined as “success, no content returned” meaning that the action completed successfully and that there’s no body. It is not specifically defined for PUT operations, however web servers[1] return this on overwrite.

[1] e.g. nginx (tested locally) and apache 2 (see https://bugs.launchpad.net/duplicity/+bug/888301)