Failure to connect to Nailgun server produces confusing error

When the NG server is not up, jruby --ng produces only the following error:

~/projects/jruby $ jruby --ng -e 1
connect: Connection refused

This error gives no indication as to the actual cause of the problem. If instead it reported "connect to Nailgun server: Connection refused" there would be less chance of confusing this error for a Ruby error.

This actually confused the Travis CI folks into thinking there was a network connectivity problem or JRuby bug affecting RubyGems or Bundler, when in actuality the problem was caused by RVM using Nailgun and (probably) not having the server initialized before the command launched. A better error message would have shown the problem very quickly.

We should fix this in the launcher, and possibly in our copy of ng client source. If we do the latter, we should also attempt to submit a patch back to the ng authors.