handle the Error data using a HttpResponseErrorHandler
default code for the error handler
better testing of Gson data
test error data that are parsed with Gson
handle Gson internally to Ion when we can
remove unused code
clean the Ion error/data handling
handle the IOException and ParserException inside the engines rather than each ErrorHandler
use shorter names it's all HTTP anyway
cleaning after renaming
rename HttpResponseHandler to ResponseHandler as it's only for HTTP
clean ResponseHandler after renaming
we don't need the HTTP request to create an exception based on the response
remove the old (unused) JSON error handling code