child_process: align fork/spawn stdio error msg

fork()'s support for .stdio strings in 3268863e used a different
TypeError string from spawn, unnecessarily.

PR-URL: https://github.com/nodejs/node/pull/11044


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>