Centralize handling of common errors

[oguzkocer]

Especially while parsing the network requests, there are a lot of errors shared between different endpoints. We should centralize these as much as possible and only handle specific errors in per-endpoint parsers.

[crazytonyli]

I thought returning HTTP 5xx indicates an programming error, not an API usage error (like passing a wrong argument). It turns out that's not the case. From my test, creating a user with an already username returns 500.

$ curl -si -u test@example.com:$(cat test_credentials) --json '{"username":"test@example.com","email":"t@t.com","password":"pass"}' 'http://localhost/wp-json/wp/v2/users'
HTTP/1.1 500 Internal Server Error
Date: Wed, 10 Apr 2024 23:48:32 GMT
Server: Apache/2.4.57 (Debian)
X-Powered-By: PHP/8.2.17
X-Robots-Tag: noindex
Link: <http://localhost/wp-json/>; rel="https://api.w.org/"
X-Content-Type-Options: nosniff
Access-Control-Expose-Headers: X-WP-Total, X-WP-TotalPages, Link
Access-Control-Allow-Headers: Authorization, X-WP-Nonce, Content-Disposition, Content-MD5, Content-Type
Allow: GET, POST
Expires: Wed, 11 Jan 1984 05:00:00 GMT
Cache-Control: no-cache, must-revalidate, max-age=0, no-store, private
Content-Length: 91
Connection: close
Content-Type: application/json; charset=UTF-8

{"code":"existing_user_login","message":"Sorry, that username already exists!","data":null}

[oguzkocer]

This was somewhat addressed in #69. Error handling is an ongoing effort and some of its design will be a result of organic growth. There is nothing particular left we have to do for 0.1, so I'll close this.