Return error if timestamp cannot be optained

Init should return an error code, if there is any error and not just
log the problem and forget about it, otherwise the user cannot handle it

src/thingsboard.c

@@ -267,6 +267,7 @@ int thingsboard_init(attr_write_callback_t cb, const struct tb_fw_id *fw_id) {
 	ret = k_sem_take(&time_sem, K_SECONDS(10));
 	if (ret < 0) {
 		LOG_ERR("Failed to wait for timestamp: %d", ret);
+		return ret;
 	}
 
 	return 0;