Allow throws_exception on a Global & Per-call Basis

* Allows `throws_exception` to be defined through a param passed to `Mailchimp::Api.new` or on a per-call basis
* Fixes broken API test
* Adds some documentation

When originally using the gem I assumed that `throws_exception` could be defined and configured in a similiar way to how `timeout` can be defined. I was a bit confused when errors were not being thrown, this PR makes the gem's options work consistently (rule of least surprise).

Looking forward to hearing your thoughts