Vague error on submit in field widget when invalid item selected

We need custom error handling when an incorrect bundle for a field is selected from the field widget.

Right now, if you have an entity browser that allows every node type, and you select one that's not allowed by the field, the entity browser doesn't prevent it, but when you go to save the form, you get a vague error, such as "This entity (node: 55759) cannot be referenced."