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.

This commit was previously reverted (see commit
0d2d903d7541070ec97028a1b201109f4070bcb3) as we had to merge into trunk
and we didn't have enough time to test it.

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