log what required key is missing

c/mainer.c

@@ -211,8 +211,10 @@ json_key_pos (int pos_obj, char *key, int required) {
 			return pos;
 		pos = json_next_pos (pos);
 	}
-	if (required)
-		die ("Required key is missing!");
+	if (required) {
+        Log("required key=%s is missing!", key);
+        die ("Required key is missing!");
+    }
 	return 0;
 }