http: Improve diagnostic messages.
* common/http.c (send_request): Print TLS alert info (connect_server): Detect bogus DNS entry.
- Prints the TLS alert description.
- Detect case where the DNS returns an IP address but the server is not reachable at this address. This may happen for a server which is reachable only at IPv6 but but the local machine has no full IPv6 configuration.