Web app: Sending test notification doesn't create proper 403 error

[cmeis]

When trying to send a test notification to a topic where the user has no write access, there's no error message at all in the GUI.

The browser console shows the 403 error not being processed:

There should be some kind of error message to the user.

[binwiederhier]

Good catch. Should be easy to add a snack bar message.

[binwiederhier]

Fixed in 448444e