CA-122253: Improve error reporting for LVM-based SR snapshots

Return the actual error instead of returning errors that might have
occurred in the error-handling path.

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
Reviewed-by: Vineeth Thampi Raveendran <vineeth.thampi@citrix.com>

GitHub: closes #60 on xapi-project/sm