There are already a lot of duplicates for this issue. See https://github.com/ValveSoftware/steam-for-linux/issues/28 for instance.
FYI: FTL was just added to the official supported game list. Please delete local content and download again.
Workaround "delete local content and download again" doesn't work at all.
On DEFCON, FTL: Faster Than Light, and Multiwinia, I get the following error:
![steamerror]()