class Async::Task

As an explcit choice, the user can start a task which doesn't propagate exceptions. This only applies to `StandardError` and derived tasks. This allows tasks to internally capture their error state which is raised when invoking `Task#result` similar to how `Thread#join` works. This mode makes {ruby Async::Task} behave more like a promise, and you would need to ensure that someone calls `Task#result` otherwise you might miss important errors.
This is a very tricky aspect of tasks to get right. I've modelled it after `Thread` but it's slightly different in that the exception can propagate back up through the reactor. If the user writes code which raises an exception, that exception should always be visible, i.e. cause a failure. If it's not visible, such code fails silently and can be very difficult to debug.

def fail!(exception = nil, propagate = true)
	@status = :failed
	@result = exception
	
	if propagate
		raise
	elsif @finished.nil?
		# If no one has called wait, we log this as an error:
		logger.error(self) {$!}
	else
		logger.debug(self) {$!}
	end
end