refget: v2 spec for Range header errors does not align with typical usage

[davidlougheed]

For range headers, it is generally accepted that the Range Not Satisfiable error is to be used if a "document doesn't contain such ranges".

In Refget v2, 400 Bad Request is used for this instead; I believe a 416 would be better because it can then require a Content-Range response header with the total sequence length, allowing clients to subsequently discover valid range queries that can be made and aligning better with the original intent of HTTP range responses.