Add member: Always return 409 when a member exists
What does this MR do?
It always returns 409 status when a member exists for a source (201 was returned for project).
Why was this MR needed?
It helps consistency in api
Does this MR meet the acceptance criteria?
- Changelog entry added
- Documentation created/updated
- API support added
- Added for this feature/bug
- All builds are passing
- Conform by the merge request performance guides
- Conform by the style guides
Branch has no merge conflicts with
master(if it does - rebase it please)
- Squashed related commits together
What are the relevant issue numbers?
Fixes #20732 (closed)