check_site: improve error message for unset values

scripts/check_site.lua

@@ -108,7 +108,12 @@ local function var_error(path, val, msg)
 		src = site_src()
 	end
 
-	config_error(src, 'expected %s to %s, but it is %s (a %s value)', path_to_string(path), msg, tostring(val), type(val))
+	local found = 'unset'
+	if val ~= nil then
+		found = string.format('%s (a %s value)', tostring(val), type(val))
+	end
+
+	config_error(src, 'expected %s to %s, but it is %s', path_to_string(path), msg, found)
 end