fix: handle Flatpak Steam game processes correctly

[ghost]

I've tested this commit, and it is working. Please read the comment, as it was a struggle to get it right.

If the game is run through Flatpak Steam, not all child processes are listed for the parent process when executing 'pgrep', so
we need to run the command with the first PID and then the output PIDs. Additionally, 'pkill' does not terminate all child
processes, so we must reverse the order so that the last process terminates first; otherwise, new untracked processes may start.

I've also replaced the exit code -11 with -15 because it didn't work, and I believe this is the correct signal to use.