add more information in case of error
add more information in case of error

cf http://lists.gforge.inria.fr/pipermail/cado-nfs-discuss/2013-November/000206.html

mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs