ADD: tell a bit more what is wrong if a multi mapping cannot be created
ADD: tell a bit more what is wrong if a multi mapping cannot be created

Former-commit-id: 860d0bfa349f7dae1259b6dba69b94bc673b4461
FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,FabienPean/sofa,hdeling/sofa,Anatoscope/sofa,Anatoscope/sofa,Anatoscope/sofa,Anatoscope/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,hdeling/sofa