Comment 2 for bug 1562645

To fix this you need to uninstall the steam-launcher package:

sudo apt remove steam-launcher

remove repo.steampowered.com from your sources and then install the steam package from multiverse:

sudo apt install steam

Once you've done this, run Steam and let it sort out its install/update/etc., then close it and move the apps and data directories to the new location:

mv ~/.steam/steamapps ~/.steam/steamapps.orig ## just in case
mv ~/.steam/userdata ~/.steam/userdata.org ## just in case

mv ~/.local/share/Steam/SteamApps ~/.steam/steamapps
mv ~/.local/share/Steam/userdata ~/.steam/userdata

You should then be able to start Steam without the error and have all your games and saves still available.

I'm not sure whether there is a practical way to do this automatically since the originally installed Steam package isn't from a Ubuntu supplied location. It might be possible to add a conflict in the steam package and automatically move the directories with a bit of experimentation.