Add error when sleep command fails (#703)

src/config/Lang.zig

@@ -36,6 +36,7 @@ err_perm_dir: []const u8 = "failed to change current directory",
 err_perm_group: []const u8 = "failed to downgrade group permissions",
 err_perm_user: []const u8 = "failed to downgrade user permissions",
 err_pwnam: []const u8 = "failed to get user info",
+err_sleep: []const u8 = "failed to execute sleep command",
 err_unknown: []const u8 = "an unknown error occurred",
 err_user_gid: []const u8 = "failed to set user GID",
 err_user_init: []const u8 = "failed to initialize user",