[PP-1] Return validation errors from LayoutController::post and ::patch

📌 Support server side massage and validation of component prop form values Active landed a while ago, this is unblocked.

In 🌱 Research: Possible backend implementations of auto-save, drafts, and publishing Active one of the reasons we discussed for an having auto-save that was independent entity revisions was to allow having saving incomplete or invalid data.

It seems this would abandon that idea or at least limit to allowing empty slots. Is that right? I think because \Drupal\experience_builder\Controller\ApiLayoutController::buildPreviewRenderable already calls \Drupal\experience_builder\ClientDataToEntityConverter::convert() the "page data" information would already be validated and we can't now store incomplete or invalid Page data values in auto-save.

While looking into this I found 🐛 XB allows saving invalid date range field values Active , but I think this because date range validation for this only happens in the field widget