After reading the further responses here I'm wondering something...
You switched to a new site, can't 301 the old pages, and have no control over the old domain... So why are you worried about pages 404ing on an unused site you don't control anymore?
Maybe I'm missing something here or not reading it right. Who does control the old domain then? Is the old domain just completely gone? Because if so, why would it matter that Google is crawling non-existent pages on a dead site and returning 404s and 500s? Why would that necessarily affect the new site?
Or is it the same site but you switched to Java from PHP? If so, wouldn't your CMS have a way of redirecting the old pages that are technically still part of your site to the newer relevant pages on the site?
I feel like I'm missing pertinent info that might make this easier to digest and offer up help.