Make i2pcontrol error codes negative.

i2pcontrol/I2PControl.cpp

@@ -39,7 +39,7 @@ std::string I2PControlSession::Response::toJsonString() const
     }
     oss << "},\"jsonrpc\":\"" << version << '"';
     if(error != ErrorCode::None)
-        oss << ",\"error\":{\"code\":" << static_cast<int>(error)
+        oss << ",\"error\":{\"code\":" << -static_cast<int>(error)
             << ",\"message\":\"" << getErrorMsg() << "\"" << "}";
     oss << "}";
     return oss.str();