Add http_status_code to ParseProtocolError.

Per our conversation at #132 

Crux of this issue is that Parse can return **HTML** (not JSON) with response code of 502 / 503 (Parse's nginx seems to be poorly configured). In this case, the only information we can reliably use is HTTP status code.

Alternatively, I would suggest that we pass `env` to the error as the initialize argument rather than parsed hash, then build the `error_hash` inside the error object, not in the `ExtendedParseJson` class.