Fix host discovery error handling

[DavidMartinezBer]

Summary

• return a UI error if mDNS host discovery cannot open its socket
• return a no-host result when discovery finishes without finding hosts
• avoid STUN lookup while processing local mDNS discovery results

Motivation

On Switch, the Add Host tab can crash or get stuck shortly after entering the tab while automatic host discovery is running. Local mDNS discovery should not perform a remote STUN lookup for every discovered LAN host, and discovery failures should be reported back to the UI instead of returning silently.

Testing

• Ran git diff --check
• CMake Switch configure could not be completed locally because DEVKITPRO is not installed in this environment