Twig syntax errors interrupt the watch task on the pattern lab server

[mathieuhelie]

When we are developing new templates while in php core/console --server --with-watch, if we save a twig template with a syntax error (or create any other kind of twig error) the watch task on the server stops working. This means correcting the error has no effect and the server needs to be restarted, making the watch task somewhat useless.