fix(frontends/lean/builtin_cmds): generate error if declaration is not a definition
fix(frontends/lean/builtin_cmds): generate error if declaration is not a definition

johoelzl/lean,fgdorais/lean,johoelzl/lean,rlewis1988/lean,rlewis1988/lean,soonhokong/lean-osx,leanprover/lean,Kha/lean,soonhokong/lean-osx,fgdorais/lean,leanprover/lean,soonhokong/lean-osx,leodemoura/lean,johoelzl/lean,johoelzl/lean,leanprover-community/lean,digama0/lean,leodemoura/lean,leodemoura/lean,Kha/lean,rlewis1988/lean,leanprover-community/lean,digama0/lean,soonhokong/lean-windows,Kha/lean,dselsam/lean,digama0/lean,leanprover-community/lean,johoelzl/lean,soonhokong/lean-osx,fpvandoorn/lean,dselsam/lean,avigad/lean,fgdorais/lean,leanprover/lean,soonhokong/lean-windows,dselsam/lean,soonhokong/lean-windows,rlewis1988/lean,fpvandoorn/lean,rlewis1988/lean,fpvandoorn/lean,leanprover/lean,soonhokong/lean-windows,johoelzl/lean,Kha/lean,leodemoura/lean,digama0/lean,fgdorais/lean,fpvandoorn/lean,leanprover/lean,rlewis1988/lean,digama0/lean,fgdorais/lean,avigad/lean,avigad/lean,dselsam/lean,leanprover-community/lean,leanprover-community/lean,soonhokong/lean-windows,leodemoura/lean,soonhokong/lean-osx,leanprover-community/lean,fpvandoorn/lean,leanprover/lean,digama0/lean,avigad/lean,dselsam/lean,fgdorais/lean,leodemoura/lean,avigad/lean,Kha/lean,fpvandoorn/lean,dselsam/lean,avigad/lean,Kha/lean