OSIS-2169 Enhanced error msg
OSIS-2169 Enhanced error msg

	modified:   base/business/group_element_years/management.py

uclouvain/OSIS-Louvain,uclouvain/osis,uclouvain/osis,uclouvain/osis,uclouvain/OSIS-Louvain,uclouvain/OSIS-Louvain,uclouvain/osis