tps-frontend: Error message contains unprocessed <tt> markup tag

To reproduce:

1. Apply this patch:

diff --git a/config/chroot_local-includes/usr/lib/python3/dist-packages/tps/configuration/feature.py b/config/chroot_local-includes/usr/lib/python3/dist-packages/tps/configuration/feature.py
index 6e8285da046..8ad81c580bd 100644
--- a/config/chroot_local-includes/usr/lib/python3/dist-packages/tps/configuration/feature.py
+++ b/config/chroot_local-includes/usr/lib/python3/dist-packages/tps/configuration/feature.py
@@ -110,6 +110,9 @@ class Feature(DBusObject, ServiceUsingJobs, metaclass=abc.ABCMeta):
     def do_activate(self, job: Optional["Job"], non_blocking=False):
         logger.info(f"Activating feature {self.Id}")
 
+        # XXX
+        executil.check_call(["touch", "/non/existent/path"])
+
         apps = self.get_running_conflicting_apps()
         if apps and non_blocking:
             # Note that we don't translate this error message because

2. Try to activate a feature in tps
3. Observe the error message containing an unprocessed <tt> tag: