Add HTTP 429 Too Many Requests as a possible generic error response

[js-nh]

Fixes #

Description of the Change

Adds 429 Too Many Requests as a possible generic error response.

Discovered during an OpenAPI schema validation run as our generic error responses were not found in the schema.

This is essentially same as #1165 but for 429 status code.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[js-nh]

There was a typo - I had a trailing , in the json structure. It should be resolved now, tests are passing locally.

[js-nh]

@sliverc Could I ask you to review this and tell me how I could proceed with getting this merged, if possible?

[sliverc]

Thanks for the addition. This looks good to me and ready to merge.

Please note though that OpenAPI schema support will be deprecated in DJA as soon as Django REST framework will deprecate it. Best take a look at #1082 for more details.

As for small fixes like this, we still integrate into DJA for the time being. I would highly recommend you though to get involved with the drf-spectecular-json-api which will be the successor of the schema generation in the near future.