[BUG] No error notification from group membership dialog

[herflis]

🐜 Bug Report

I've tested the new builtin user and group protection feature and it turned out that there is no error notification when an error occurs in the group membership dialog.

📦 Package / Application

@sensenet/sn-app

📃 Version

latest (develop)

🐜 Describe the bug

There are cases when a user could not be removed from a group and the backend throws an error but it is not handled on the client.

👉 Steps To Reproduce

1. Login to the netcore test repository on the admin-ui with businesscat
2. Go to Users & Groups and switch to groups
3. Click on the member button at the administrators group's row
4. Try to remove businesscat

😯 Current behavior

No notification with the error message is shown after the request.

🤔 Expected behavior

A notification should appear with the message of the actual error.