Do not warn about local address failures on every update

localAddrs runs inside every SendUpdate, which fires on lighthouse.interval
and again on every network change via RebindUDPServer. Logging the failure
there meant a persistent failure warned every interval for the life of the
process, once per failing interface.

collectLocalAddrs now returns its failures instead of logging them, which
keeps it stateless and lets the tests assert on errors rather than log
output. The lighthouse holds the previous error and warns only when it
changes, demoting repeats to Debug, matching how handshake_manager handles
repeated send failures.

hostmap.go

@@ -868,26 +868,26 @@ func (i *HostInfo) logger(l *slog.Logger) *slog.Logger {
 
 // Utility functions
 
-func localAddrs(l *slog.Logger, allowList *LocalAllowList) []netip.Addr {
+func localAddrs(l *slog.Logger, allowList *LocalAllowList) ([]netip.Addr, error) {
 	return collectLocalAddrs(l, allowList, localInterfaces, localInterfaceAddrs)
 }
 
 // collectLocalAddrs takes its enumerators as arguments so tests can drive the filtering and the
-// failure branches without depending on the addresses of whatever host they run on.
+// failure branches without depending on the addresses of whatever host they run on. It reports
+// failures to the caller rather than logging them, because it runs on every lighthouse update and
+// only the caller can tell a new failure from a repeat of the same one.
 func collectLocalAddrs(
 	l *slog.Logger,
 	allowList *LocalAllowList,
 	interfaces func() ([]net.Interface, error),
 	interfaceAddrs func(*net.Interface) ([]net.Addr, error),
-) []netip.Addr {
+) ([]netip.Addr, error) {
 	//FIXME: This function is pretty garbage
 	var finalAddrs []netip.Addr
+	var errs []error
 	ifaces, err := interfaces()
 	if err != nil {
-		l.Warn("Failed to enumerate local interfaces, no underlay addresses will be advertised to lighthouses",
-			"error", err,
-		)
-		return nil
+		return nil, fmt.Errorf("failed to enumerate local interfaces: %w", err)
 	}
 
 	for _, i := range ifaces {
@@ -904,10 +904,7 @@ func collectLocalAddrs(
 		}
 		addrs, err := interfaceAddrs(&i)
 		if err != nil {
-			l.Warn("Failed to get addresses for local interface",
-				"error", err,
-				"interfaceName", i.Name,
-			)
+			errs = append(errs, fmt.Errorf("failed to get addresses for %s: %w", i.Name, err))
 			continue
 		}
 
@@ -945,5 +942,5 @@ func collectLocalAddrs(
 			}
 		}
 	}
-	return finalAddrs
+	return finalAddrs, errors.Join(errs...)
 }