tools: fix exit code when linting from CI

Before this, if there were lint errors reported by `make jslint-ci`,
the process would still exit with an exit code of zero.

This commit fixes that to align with `make jslint` (exit with
non-zero on lint errors).

PR-URL: https://github.com/nodejs/node/pull/6412


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Phillip Johnsen <johphi@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>