sheepdog: Fix misleading error messages in sd_snapshot_create()

If do_sd_create() fails, it first reports the error returned, then
reports a another one with strerror(errno).  errno is meaningless at
that point.

Report just one error combining the valid information from both
messages.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Liu Yuan <namei.unix@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
