cmd/cli: only emit error for running interfaces

While at it, also ensure setDNS/resetDNS return a wrapped error on
Darwin/Windows, so the caller can decide whether to print the error to
users.

cmd/cli/cli.go

@@ -2223,3 +2223,8 @@ func absHomeDir(filename string) string {
 	}
 	return filepath.Join(dir, filename)
 }
+
+// ifaceUp reports whether the net interface is up.
+func ifaceUp(iface *net.Interface) bool {
+	return iface != nil && iface.Flags&net.FlagUp != 0
+}