Entity is not validated prior to being saved

Going further on the patch in #4:

Views_bulk_operations expects $result['message'] to be a MarkupInterface. I changed the patch for this and added the type "error" so the messages get the correct status.

Expanded the patch to limit validation errors to only the ones that the user is able to edit, consistent with content entity forms (https://git.drupalcode.org/project/drupal/-/blob/11.x/core/lib/Drupal/Core/Entity/ContentEntityForm.php?ref_type=heads#L212)

A small improvement in that the field label is nog also mentioned in the error message, which makes it easier for the user to know which field is causing the error message. As mentioned before, the error reporting is still very much barebones and I expect the work done in #1327632: Support action specific status message reporting. → could help us here.