Improve wording for blocked member promotions due to seat overages

Paulo Barrosrequested to merge
pb/498728/improve-wording-user-promotion-bso into master

Resolves Follow-up from "Handle Member Promotions When B... (#498728).

What does this MR do and why?

This MR improves the error message shown when a member promotion is rejected due to Restricted Access being enabled.

The message changes from "No seat available" to: "Could not update role because there are no available seats in this group".

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