Fix handling of errors in aliases CSV upload (#1838)

* Fix handling of errors in aliases CSV upload

* changelog

Author: davisagli

news/1837.bugfix

@@ -0,0 +1 @@
+URL Management control panel: Fix error handling in CSV upload. @davisagli

src/plone/restapi/services/aliases/add.py

@@ -101,12 +101,22 @@ def _reply_csv(self):
             raise BadRequest("Uploaded file is not a valid CSV file")
 
         controlpanel = RedirectsControlPanel(self.context, self.request)
+        csv_errors = controlpanel.csv_errors = []
         storage = getUtility(IRedirectionStorage)
         status = IStatusMessage(self.request)
         portal = getSite()
         controlpanel.upload(file, portal, storage, status)
         file.close()
 
+        if csv_errors:
+            self.request.response.setHeader("Content-Type", "application/json")
+            self.request.response.setStatus(BadRequest)
+            return {
+                "type": "BadRequest",
+                "message": f"Found {len(csv_errors)} errors in CSV file.",
+                # Skip first item which is a notice about the delimiter
+                "csv_errors": csv_errors[1:],
+            }
         if err := status.show():
             if err[0].type == "error":
                 raise BadRequest(err[0].message)