add special handling for exceptions in JSON format

[ericphanson]

based on https://github.com/beacon-biosignals/BeaconK8sUtilities.jl/pull/14

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[ericphanson]

with recursive JSON we could do something like emit a list of stackframes and maybe you could do some analysis to see what lines are showing up the most or something over a large codebase... But probably that's generally not going to be used, and you can always just use a different key like stacktrace = jsonify(stacktrace(catch_backtrace())) or something if you do want that (for some appropriate jsonify).