add better error messages when request has a problem

before this commit, all errors from node-oss-client looked like:

    DEBUG: Starting child process with 'node server'
    Error: connect ECONNREFUSED
        at errnoException (net.js:901:11)
        at Object.afterConnect [as oncomplete] (net.js:892:19)

Not very usefull. With this commit, we add better stack trace and
better error messages on common errors like server not available.
Merge pull request #4 from vvo/better-error-message-on-connect-issues

add better error messages when request has a problem