Fix logrotate's postrotate script - it shouldn't fail when the process in the PID file isn't running

We should fail gracefully if the process referenced in the pid file isn't actually running. This solves the nightly cron error we were getting:

/etc/cron.daily/logrotate:
logrotate_script: 2: kill: No such process

error: error running shared postrotate script for '/var/log/sensu/sensu-client.log '
run-parts: /etc/cron.daily/logrotate exited with return code 1