Better error message when unknown closure is used
Better error message when unknown closure is used

brechtvl/OpenShadingLanguage,gzorin/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,aconty/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,aconty/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,lgritz/OpenShadingLanguage,imageworks/OpenShadingLanguage,aconty/OpenShadingLanguage,lgritz/OpenShadingLanguage,svenstaro/OpenShadingLanguage,imageworks/OpenShadingLanguage,gzorin/OpenShadingLanguage,brechtvl/OpenShadingLanguage,lgritz/OpenShadingLanguage,brechtvl/OpenShadingLanguage,svenstaro/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,brechtvl/OpenShadingLanguage,aconty/OpenShadingLanguage,gzorin/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,lgritz/OpenShadingLanguage,svenstaro/OpenShadingLanguage,svenstaro/OpenShadingLanguage,gzorin/OpenShadingLanguage,imageworks/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,aconty/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,fpsunflower/OpenShadingLanguage,amartinez-cg/OpenShadingLanguage,imageworks/OpenShadingLanguage,imageworks/OpenShadingLanguage,gzorin/OpenShadingLanguage