Improve error codes for room directory

From discussions I had in https://github.com/vector-im/riot-web/issues/13661 when user add a Matrix instance to look for chans/rooms in there through the "Explore rooms feature", like in Riot, it seems the response the client get isn't really detailed, and basically is just a "I couldn't contact the server".

I don't know very well Matrix servers and mechanics, but from this discussion, I distinguish at least two different cases that would deserve a different response from server, allowing for clients to give their users a more accurate information:
- If the server was indeed unreachable (offline, bad name so no existing server. Those two might even be separated if we consider if the DNS record exist or not)
- If the server was reached, but the directory listing isn't public. This way, the user wouldn't be confused whether their input is correct or not.

I might have not considered other cases due to my lack of knowledge on possible situations.

Start by reading the linked Riot issue, matrix-org/riot-web#13661, and compare its discussion with the Matrix room-directory behavior described here. Identify where the specification defines responses for unreachable servers and non-public directories. Done means the specification clearly distinguishes these cases or documents the agreed error-code design.

Written by the indexing model from the issue text.