Removal of ResponseErrorHandler

[k9withabone]

I'm thinking about removing ResponseErrorHandler and it's corresponding function arguments that are in every endpoint, stemming from AudiobookshelfApi.requestJson(). This would also make the return types of those functions non-nullable.
The original idea of ResponseErrorHandler was to provide a place where the caller could have access to the original http.Response when doing JSON conversion and something went wrong. An alternative solution for this is to catch all throws from fromJson(), wrap it in a new error type that also includes the http.Response, and throw that.

@keaganhilliard, I would like your thoughts on this.