Improve stratum error handling #62

I started to submit invalid requests to the Stratum port to see how it handles them. Not horrible, but not great either.

curl --data '{"id": 1, "method": "mining.subscribe", "params": ["CONNECT_HOST", CONNECT_PORT, "MINER_USER_AGENT", "SESSION_ID"]}'  http://localhost:19031

returns this:

{"result":null,"error":{"code":-32700,"message":"Invalid JSON, Parse error on: POST / HTTP/1.1"},"id":null}
{"result":null,"error":{"code":-32700,"message":"Invalid JSON, Parse error on: Host: localhost:19031"},"id":null}
{"result":null,"error":{"code":-32700,"message":"Invalid JSON, Parse error on: User-Agent: curl/7.64.0"},"id":null}
{"result":null,"error":{"code":-32700,"message":"Invalid JSON, Parse error on: Accept: */*"},"id":null}
{"result":null,"error":{"code":-32700,"message":"Invalid JSON, Parse error on: Content-Length: 115"},"id":null}
{"result":null,"error":{"code":-32700,"message":"Invalid JSON, Parse error on: Content-Type: application/x-www-form-urlencoded"},"id":null}
{"result":null,"error":{"code":-32700,"message":"Invalid JSON, Parse error on: "},"id":null}

and also hangs until a CTRL-C is given. We should abort on the first parse error, we should not parse HTTP headers as json and close the connection as well. And also log the invalid request in debug.log.