That's kind of ugly ;-)  It would make it look like the "ERROR: ..." line is the most recent line from the Chef run.  How about this:

    def get_log_lines(pids, lines)
      logfile = "/var/log/crowbar/chef-client/#{pids}.log"
      lines = `tail -n #{lines} #{logfile}`
      if $?.success?
        "Most recent logged lines from the Chef run: \n" + lines
      else
        "ERROR: Couldn't read #{logfile}"
      end
    end