Better error handling

If you use the the module without the internet disconnected, there will be no response object. So, I tried to fix that.

This was the error you get when you try to run the module without internet. Not so pretty 👎

if (response.statusCode > 399) {
                        ^
TypeError: Cannot read property 'statusCode' of undefined
    at Request._callback (/Users/candh/Desktop/test/node_modules/node-genius/lib/index.js:21:25)
    at self.callback (/Users/candh/Desktop/test/node_modules/request/request.js:186:22)
    at emitOne (events.js:96:13)
    at Request.emit (events.js:188:7)
    at Request.onRequestError (/Users/candh/Desktop/test/node_modules/request/request.js:845:8)
    at emitOne (events.js:96:13)
    at ClientRequest.emit (events.js:188:7)
    at TLSSocket.socketErrorListener (_http_client.js:310:9)
    at emitOne (events.js:96:13)
    at TLSSocket.emit (events.js:188:7)