Handle thread shutdown responses via low level error messages

[zanker-stripe]

Description

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This is a follow on from the suggestions in #2182 (comment). I did not make the change around DefaultLowlevelErrorHandler, because:

puma/lib/puma/server.rb

Lines 819 to 823 in 0b737cc

	if @leak_stack_on_error
	[500, {}, ["Puma caught this error: #{e.message} (#{e.class})\n#{e.backtrace.join("\n")}"]]
	else
	[500, {}, ["An unhandled lowlevel error occurred. The application logs may have details.\n"]]
	end

The leak_stack_on_error option is stored on the server instance and not options, which leaves no way of getting that data into the handler. I wasn't a fan of passing that through, and reorganizing how leak_stack_on_error works also felt out of scope.

Functionality wise, there's no changes besides thread shutdown now goes through lowlevel error handler and we pass status to the handler based on arity. If leak_stack_on_error is set, we include the stack, if not we show the generic error messages.

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[zanker-stripe]

Technically this means we pass env on shutdown errors, but that seemed like more of an oversight than an initial design. Happy to unwind if I'm wrong though.

[nateberkopec]

1 more thing then this is g2g for me.

[zanker-stripe]

Thanks! Changes made.