Better errors when version is not available

[michaelsauter]

I updated deps manually and run nitrogen -i. However, the specified commit was not downloaded yet. I think nitrogen just quit early. It would be great to display an error or run go get -uautomatically.

[lucachr]

Yes, you're right. I think that displaying an error is enough, so that the user can choose what is the most appropriate thing to do. Thank you for your suggestion!