LU-3435 util: Improvement to error logging for l_getidentity

LU-3435 util: Improvement to error logging for l_getidentity

The LU-2107 patch to l_getidentity does not set the data->idd_err
value to errno, and does not log the uid when getgrouplist() fails.
This patch fixes both of those concerns.

Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I8a5715bada61a4c69d4d875b60097f9414a25f1c
Reviewed-on: http://review.whamcloud.com/6533
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>