fix(shell/lean): nicer error message for file not found
fix(shell/lean): nicer error message for file not found

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