add configuration option to allow throwing of errors instead of objects

[wodka]

fixes #21

Idea is that we can configure this behaviour:

const c = new ChargeBee()

c.configure({
  site: '***',
  api_key: '***',
  useErrorObject: true
})

[ibratoev]

It is a bit strange having an option to throw errors instead of objects. What is the point of throwing objects at all?
It is a standard JS practice of using Error for... errors.

[wodka]

I'm for only throwing errors!