def check_existing_pid
      return false unless pid_file_exists?
      pid = read_pid
      fail 'Existing process found.' if pid > 0 && pid_exists?(pid)
      delete_pid
    end
def check_existing_pid
      return false unless pid_file_exists?
      pid = read_pid
      fail 'Existing process found.' if pid > 0 && pid_exists?(pid)
      delete_pid
    end
Check if process exists then fail, otherwise clean up.
 @return [Boolean] `false` if no PID file exists, `true` if it cleaned up.