Comment by salmonellaeater

A useful error message would have made this a 1-minute investigation. The "fix" of trying to detect this specific program is much too narrow. The right fix is to change Yarn to print a message about what it was trying to do (check for a Sapling repo) and what happened instead. This is also likely a systemic problem, so a good engineer would go through the whole program and fix other places that need it.