text: Title: 🐛fix some error messages
  Body: ## Bug Report
  
  To display some error messages, we are doing a condition on the error message that we get from the backend. It is error prone because the backend error message are internationalized.
  
  ## What to do
  
  Find another way to display clean error message (code status ?).
  
  ## Code location
  
  - [ ] Fix on AddMembers.tsx
  https://github.com/numerique-gouv/impress/blob/60120852f502c40da961e07110a7cdafc5c1b3e0/src/frontend/apps/impress/src/features/docs/members/members-add/components/AddMembers.tsx#L88-L104
  
  - [x] Fix on AIButton.tsx
  https://github.com/numerique-gouv/impress/blob/60120852f502c40da961e07110a7cdafc5c1b3e0/src/frontend/apps/impress/src/features/docs/doc-editor/components/AIButton.tsx#L365-L374