Display debug-on-exception messages

[yuhan0]

Support for https://github.com/clojure-emacs/cider-nrepl/pull/772

A quick PR without the proper changelogs / updates to manual, etc. Changes untested after being rebased on master branch, but I've been dogfooding them on a messy local branch with many (hopefully unrelated) changes for the last month or two. Might be able to clean things up a bit this weekend.

Does not provide feature for auto-inserting #dbg! macros or instrumenting defns (eg. with negative argument) - see #3337

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Thanks!

If you're just starting out to hack on CIDER you might find this section of its manual extremely useful.