assert: support custom errors

This commit adds special handling of Error instances when passed
as the message argument to assert functions. With this commit,
if an Error is passed as the message, then that Error is thrown
instead of an AssertionError.

PR-URL: https://github.com/nodejs/node/pull/15304


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>