net/dns: return error from NewOSManager, use it to initialize NM.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/resolved.go

@@ -87,10 +87,10 @@ type resolvedManager struct {
 	logf logger.Logf
 }
 
-func newResolvedManager(logf logger.Logf) resolvedManager {
+func newResolvedManager(logf logger.Logf) (resolvedManager, error) {
 	return resolvedManager{
 		logf: logf,
-	}
+	}, nil
 }
 
 // Up implements managerImpl.