Support error reporting from serialization (binary & JSON)

[tribal-tec]

Requires changes on the SerializeCallback and DeserializeCallback to have either return values or in-out parameters with enum for error code and an optional error message. This way, applications can add error reporting that is eventually transported to an HTTP client for instance.

namespace servus
{

...

// in the spirit of HTTP status codes
enum class Status
{
    OK,
    BAD_REQUEST,
    ...
}

struct Response
{
    Status status = OK;
    std::string message;
}

typedef std::function< void( Response& ) > DeserializedCallback;
typedef std::function< void( Response& ) > SerializeCallback;

}