The Yellowstone Caldera is big, but it’s not that big. Even the US East of the Mississippi would mostly be okay (dealing with an inch or two of ash), let alone the rest of the world. There’d be a volcanic winter worse than 1815, sure, but between modern farming and logistics and the fact that it would maybe even help a little bit by temporarily mitigating global warming, I wouldn’t expect an “agonizing death [of] the rest of the world” level of famine.