enhance: improve admin UI permissions error text

[wesleyboar]

Overview

Improve confusing text error message in the Django CMS Admin UI.

Related

None.

Backstory

When trying to give staff member Publish permission on ECEP, I hit confusing error message.

^{I got angry at Django CMS. I thought about proposing better error text on Django CMS proper. Then I realized I don't know if I should edit their msgstr in their en po file, or just change in my own app. I remembered how much all of Django CMS text sucks, but also how unfamiliar I am with what most of it means, and did not trust myself to improve Django CMS at the source. Especially, because I am out-of-date with their interface (we run older version of their software).}

Changes

• changed four error messages.

Testing

1. Open http://0.0.0.0:8000/admin/cms/globalpagepermission/add/.
2. For any Group or User, give/check only the permission "Can publish".

UI

before	after

[taoteg]

LGTM! Clearer action for users.