cluster: expose result of send()

There are several places in the cluster module where a version
of process.send() is called, but the result is swallowed. Most
of these cases are internal, but Worker.prototype.send(), which
is publicly documented, also suffers from this problem. This
commit exposes the return value to facilitate better error
handling, and bring Worker.prototype.send() into compliance
with the documentation.

PR-URL: https://github.com/nodejs/node/pull/6998


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>