I’ve never used Symfony, so I have no idea how you’d handle that on the back end. The error handling doesn’t sound like a Turbo or Stimulus-specific problem, though. This is more basic application logic. If you submit a regular form and there are errors, how are you handling them?
You can check out this post for inspiration about handling form submissions, even if it’s with Rails.
Really, though, if you need help, it makes sense to post some code you’ve tried. What you’ve given us so far are just specs.