OSIS-923 Corrected exception catch
OSIS-923 Corrected exception catch

	modified:   base/models/proposal_learning_unit.py

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