--- a/util/promise.lua	Fri Oct 26 09:23:31 2018 +0100
+++ b/util/promise.lua	Fri Oct 26 16:09:25 2018 +0100
@@ -15,7 +15,7 @@
 		return default;
 	end
 	return function (param)
-		local ok, ret = pcall(f, param);
+		local ok, ret = xpcall(f, debug.traceback, param);
 		if ok then
 			resolve(ret);
 		else