PE-2172: don't call System/exit

This is a first-cut and I'm sort of soliciting feedback here; the main issue is: what structure do you wanna throw?  Right now, this just throws a string which is intended to be displayed to the user before terminating the process.  This results in code like
```clojure
(try+
  ;; do stuff ...

  (catch string? msg
    (println msg)
    (System/exit 0)))
```

The other idea I had is to throw a map with a specific, documented key for the error message.