(#6337) Fix Ruby warning on 1.8.6 about "future compatibility"

Ruby 1.8.6 (but not later versions) warn about requiring parenthesis on some
function calls; having one of those in our network rights checking means that
we emit ... quite a few of these, and annoy anything that tracks our logs.

By using the more standard form of raise we can avoid the warning entirely,
and keep consistent code style across the file.

Reviewed-By: Paul Berry <paul@puppetlabs.com>