improve error messages on unique value validation

Author: cekk

src/design/plone/policy/restapi/submit_form/post.py

@@ -47,13 +47,20 @@ def get_form_data(self):
                     submit_value = submitted_data.get(field_id, {}).get("value", None)
                     if submit_value and submit_value == saved_value:
                         raise BadRequest(
-                            api.portal.translate(
-                                _(
-                                    "save_data_exception",
-                                    default='Unable to save data. The value of field "${field}" is already stored in previous submissions.',
-                                    mapping={"field": label},
-                                )
-                            )
+                            [
+                                {
+                                    "message": api.portal.translate(
+                                        _(
+                                            "save_data_exception",
+                                            default='Unable to save data. The value of field "${field}" is already stored in previous submissions.',
+                                            mapping={"field": label},
+                                        )
+                                    ),
+                                    "field_id": field_id,
+                                    "label": label,
+                                    "error": "UniqueValueError",
+                                }
+                            ]
                         )
 
         return form_data

src/design/plone/policy/tests/test_limit_submit_form.py

@@ -187,5 +187,5 @@ def test_unique_field(self):
         message = response.json()["message"]
         self.assertEqual(
             message,
-            'Unable to save data. The value of field "Name" is already stored in previous submissions.',
+            "[{'message': 'Unable to save data. The value of field \"Name\" is already stored in previous submissions.', 'field_id': 'name', 'label': 'Name', 'error': 'UniqueValueError'}]",
         )