assert: accommodate ES6 classes that extend Error

`assert.throws()` and `assert.doesNotThrow()` blow up with a `TypeError`
if used with an ES6 class that extends Error.

Fixes: https://github.com/nodejs/node/issues/3188
PR-URL: https://github.com/nodejs/node/pull/4166


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>