domain,events: support non-object 'error' argument

Fix a TypeError when emitting an 'error' argument with a non-object
argument (like a string) when domains are active.

Fixes: https://github.com/nodejs/help/issues/501
PR-URL: https://github.com/nodejs/node/pull/11438


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>