Node.js API Module - Improve Error Messages/Events

The wrapper for using AeroGear's API via Node.js could return better responses on errors. I recently had an issue whereby passing the alias as a Number rather than String resulted in a 500, but it took a while to figure out since all I knew was that a 500 had been triggered.

Maybe update this to return the request body (although I'm not sure what the body would have been in this case): https://github.com/aerogear/aerogear-unifiedpush-nodejs-client/blob/master/lib/unifiedpush-node-sender.js#L45