Multibyte message is not displayed properly on exception

There is a regression in 3.1.10 which affects all boards with non-latin multibyte messages.
This bug was added with this commit https://github.com/phpbb/phpbb/commit/23bdb2eedebee5a625ba35baf3098566bb966127
The 4th parameter to set_var must be true to properly display multibyte messages.