lake update nonexistant-pkgs does not complain

I just got confused for a while, because I gave the wrong package name to `lake update`. It seems that

* `lake update does-not-exist` does not print an error message
* package names are case sensitive, so typos are even more likely


It would be a small QoL improvement if `lake update pkg` would err if `pkg` is not something we can `update`.