(TK-487) Allow friendly init/start fail fast via ::exit throw

[rbrw]

Allow init and start methods to throw a request-shutdown style ex-info map to short circuit the startup process and exit with a specified message and status, rather than a backtrace.

This just provides a short circuiting (immediate) counterpart to the existing, deferred shutdown requests provided by request-shutdown.

[rbrw]

This isn't quite ready (e.g. at least needs tests), but I wanted to see if it seemed plausible before proceeding further. The current motivation is wanting to be able to shut down in a friendly way when a service detects a problem with the configuration it retrieves via (get-config), without having to carefully coordinate with all other services in order to safely rely on a deferred request-shtudown.

[puppetcla]

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