The HyperText Transfer Protocol (HTTP)
422 Unprocessable Entity response status code indicates
that the server understands the content type of the request entity, and the syntax of
the request entity is correct, but it was unable to process the contained instructions.
Important: The client should not repeat this request without modification.
422 Unprocessable Entity
|RFC 4918, section 11.2: 422 Unprocessable Entity||HTTP Extensions for Web Distributed Authoring and Versioning (WebDAV)|