Fixes #196172: manual IID should render conflict

What does this MR do?

This MR fixes #196172 (closed)

The change in this merge request only catches a given exception (::ActiveRecord::RecordNotUnique) and gives a different API response when this is catched.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

This is the first code I contribute to gitlab (and my first ruby code). I don't yet know where to add tests or something, but would be happy to learn it and update this merge request.

Since the change seems trivial, I don't expect any risks due to this change.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.