discovery: don't panic on libmdns errors

On panic, the discovery task crashes, but the main program is not
notified of this. Returning an error will result in the Discovery stream
yielding None, serving as notification to the application (which might
shutdown with error, for example, if no other means of authentication is
available).

discovery/src/lib.rs

@@ -396,17 +396,17 @@ fn launch_libmdns(
 
     let task_handle = tokio::task::spawn_blocking(move || {
         let inner = move || -> Result<(), DiscoveryError> {
-            let svc = if !zeroconf_ip.is_empty() {
+            let responder = if !zeroconf_ip.is_empty() {
                 libmdns::Responder::spawn_with_ip_list(
                     &tokio::runtime::Handle::current(),
                     zeroconf_ip,
                 )
             } else {
                 libmdns::Responder::spawn(&tokio::runtime::Handle::current())
             }
-            .map_err(|e| DiscoveryError::DnsSdError(Box::new(e)))
-            .unwrap()
-            .register(
+            .map_err(|e| DiscoveryError::DnsSdError(Box::new(e)))?;
+
+            let svc = responder.register(
                 DNS_SD_SERVICE_NAME.to_owned(),
                 name.into_owned(),
                 port,