Distinguish when a Tor HS is "not found" vs "not reachable" (exists / does not exists)

Tor2web software (but also on other software) need to provide a different response depending if a Tor Hidden Service that a user is trying to access exists or not.

The error handling revealed that today there's no specific return code to distinguish between:

• TorHS does not exists
• TorHS exists (and then it maybe reachable or not)

So in order to mimic the error-response typical of internet environment:

• Host not found (TorHS does not exists)
• Host unreachable (Tor HS exists but is not reachable)

After a discussion on IRC with Nick and sebastian the following conclusion arised:

nickm: so, this concept of ==== "We can have "exists" mean "This HS has been running recently, and has been attempting to provide service", though and implement that by saying that an HS "exists" if we find a descriptor for it, and doesn't exist if we don't. ==== is already "available" os must be implemented?

nickm naif: I don't think that's exposed from Tor right now.

This ticket is to provide an implementation for that feature.

Whenever possible we should use a Return Code from Socks Server following SOCKS protocol so that a Socks Client can parse the result and know that connection to this TorHS is "Not Found".