express, koa: don't continue past playground render

[wmertens]

Fixes #1420

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[evans]

@wmertens Thank you so much! Turns out we had the issue in koa too, so I went ahead and made that fix