Introduce a new http-completion-state when the response is not able to be parsed

[davidradl]

At the moment with continue-on-error if the response is not able to be deserialised / parsed, then we get an http-completion-state = EXCEPTION with the error_string as the the Exception message.

We would like to improve this by having a new enum value to http-completion-state
UNABLE_TO_PARSE_RESPONSE in this case the error-string will be the response body that we could not parse.