chore(frontends/lean/builtin_exprs): update error messages for do-blocks
chore(frontends/lean/builtin_exprs): update error messages for do-blocks

leodemoura/lean,avigad/lean,fpvandoorn/lean,digama0/lean,rlewis1988/lean,avigad/lean,leanprover-community/lean,johoelzl/lean,rlewis1988/lean,rlewis1988/lean,fpvandoorn/lean,leanprover-community/lean,fpvandoorn/lean,rlewis1988/lean,leanprover/lean,leodemoura/lean,leanprover/lean,digama0/lean,leanprover-community/lean,fpvandoorn/lean,leodemoura/lean,Kha/lean,leodemoura/lean,dselsam/lean,dselsam/lean,johoelzl/lean,dselsam/lean,leanprover-community/lean,Kha/lean,leanprover/lean,Kha/lean,leanprover/lean,fpvandoorn/lean,dselsam/lean,dselsam/lean,dselsam/lean,johoelzl/lean,avigad/lean,leodemoura/lean,avigad/lean,johoelzl/lean,digama0/lean,fpvandoorn/lean,digama0/lean,digama0/lean,avigad/lean,leanprover/lean,digama0/lean,avigad/lean,Kha/lean,Kha/lean,leanprover-community/lean,rlewis1988/lean,leodemoura/lean,johoelzl/lean,leanprover/lean,Kha/lean,rlewis1988/lean,leanprover-community/lean,johoelzl/lean