fix(frontends/lean/parser): generate error when 'exit' command is used
fix(frontends/lean/parser): generate error when 'exit' command is used

m_theorem_queue.join() method assumes there are no open namespaces/scopes

leodemoura/lean,levnach/lean,leodemoura/lean,johoelzl/lean,soonhokong/lean-windows,fgdorais/lean,fpvandoorn/lean2,soonhokong/lean-osx,johoelzl/lean,sp3ctum/lean,soonhokong/lean-osx,javra/lean,htzh/lean,eigengrau/lean,dselsam/lean,htzh/lean,c-cube/lean,eigengrau/lean,UlrikBuchholtz/lean,leanprover-community/lean,soonhokong/lean-windows,sp3ctum/lean,fpvandoorn/lean2,soonhokong/lean,fpvandoorn/lean2,rlewis1988/lean,htzh/lean,fpvandoorn/lean,levnach/lean,dselsam/lean,UlrikBuchholtz/lean,fgdorais/lean,johoelzl/lean,avigad/lean,digama0/lean,johoelzl/lean,levnach/lean,dselsam/lean,fgdorais/lean,johoelzl/lean,leanprover-community/lean,levnach/lean,fpvandoorn/lean,leodemoura/lean,soonhokong/lean-osx,fpvandoorn/lean2,dselsam/lean,sp3ctum/lean,fpvandoorn/lean,soonhokong/lean-osx,soonhokong/lean,c-cube/lean,javra/lean,dselsam/lean,soonhokong/lean,avigad/lean,leanprover-community/lean,eigengrau/lean,leanprover/lean,rlewis1988/lean,johoelzl/lean,rlewis1988/lean,soonhokong/lean,leodemoura/lean,Kha/lean,Kha/lean,Kha/lean,c-cube/lean,eigengrau/lean,Kha/lean,leanprover-community/lean,c-cube/lean,fpvandoorn/lean2,leanprover/lean,UlrikBuchholtz/lean,avigad/lean,soonhokong/lean,digama0/lean,soonhokong/lean-osx,fgdorais/lean,leanprover-community/lean,soonhokong/lean-windows,fgdorais/lean,javra/lean,soonhokong/lean-windows,dselsam/lean,digama0/lean,leanprover/lean,Kha/lean,eigengrau/lean,UlrikBuchholtz/lean,leodemoura/lean,Kha/lean,leanprover-community/lean,leanprover/lean,javra/lean,rlewis1988/lean,leanprover/lean,fpvandoorn/lean,rlewis1988/lean,avigad/lean,c-cube/lean,htzh/lean,fpvandoorn/lean,leodemoura/lean,avigad/lean,leanprover/lean,levnach/lean,digama0/lean,javra/lean,soonhokong/lean-windows,rlewis1988/lean,digama0/lean,UlrikBuchholtz/lean,avigad/lean,fgdorais/lean,htzh/lean,sp3ctum/lean,sp3ctum/lean,fpvandoorn/lean,digama0/lean