Exit with failure code when sidekiq-cluster child process fails

sidekiq-cluster handles process supervision for its child Sidekiq
processes, and terminates itself and all child processes if any child
Sidekiq process exits.

Previously, it always exited with a 0 status code (i.e. success), no
matter how the child process had terminated.

Now it exits with 1 if any child process had a non-zero exit code. This
allows a process supervisor one level up (like systemd) to detect
failures and restart.

Changelog: changed