[FIX] http_routing: hide the traceback by default on error 500 page

Before this commit, the error 500 page would always shows the full traceback
even if it is supposed to only show it in debug mode (or if editable).

That behavior was probably broken by the debug refactoring [1], which
introduced side effect with [2] (see template `http_error` which was moved with
[3]).

[1]: https://github.com/odoo/odoo/commit/d7cd97a9be63ae0a8643400b56d6f9a03285fba6
[2]: https://github.com/odoo/odoo/commit/24a20cd3fb00657da44d10a7a7d89cf94bdbdd83
[3]: https://github.com/odoo/odoo/commit/423402f1e50000186924186beba0cbe87e02e774

Part-of: odoo/odoo#77906