HTTP Status code should be a 400 instead of a 409 when attempting to add a term (by slug) that already exist

[shooper]

The patch to address #41370, which released in WP 4.9 changed the HTTP status code returned when a term already exists to 409. It should have been an HTTP 400 for consistency with the rest of the endpoints.

[rachelbaker]

In 42354:

REST API: Correct HTTP status code in error for requests to create a duplicate term.

The 409 error code is intended for situations where it is expected that the user will resolve the conflict and resubmit the same request. We use 400 error codes for other routes when a duplicate request is made. The 400 status code tells the user they need to modify their request for it to be successful.

Props shooper.
Fixes #42781. See #41370.

[SergeyBiryukov]

[42354] should be backported to the 5.0 branch.

[danielbachhuber]

In 43756:

REST API: Correct HTTP status code in error for requests to create a duplicate term.

The 409 error code is intended for situations where it is expected that the user will resolve the conflict and resubmit the same request. We use 400 error codes for other routes when a duplicate request is made. The 400 status code tells the user they need to modify their request for it to be successful.

Props shooper.
Merges [42354] to the 5.0 branch.
Fixes #42781. See #41370.