test: improve assert message

Improves the message when an assertion fires in the
test-net-pipe-connect-errors so that it indicates the incorrect value
received rather than merely reporting that the value is incorrect.

PR-URL: https://github.com/nodejs/node/pull/4461


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>