Allow shutting down with non-zero exit status, but no backtrace

This is useful when a program has already reported a friendly error message on say stderr and/or to the log. The issue arose when puppetdb wanted to initiate a shutdown after detecting that the currently available database was unsupported.

I could imagine allowing an exit status argument to (shutdown ...), though I suppose we'd have to define how competing requests are handled.

Another option might be support for throwing something like

(ex-info ... {:kind ::tk/notify-and-exit :status 2 :message "something for *err*, etc."})