Change API error response to focus on singular errors

[rmccue]

(Migrating ​https://github.com/WP-API/WP-API/issues/1473 here.)

Basically: right now, the API returns a list of objects on error:

[
    {
        "code": "rest_no_route",
        "message": "No route was found matching the URL and request method",
        "data": {
            "status": ​404
        }
    }
]

This is because under the hood, WP_Error holds multiple errors. Most people don't use this functionality at all, and this complicates error handling on the client site.

This is p late, but still early enough to change this IMO. Needs sign-off though, since it's technically a BC break, and it cuts out some potential use cases of WP_Error

[dd32]

Since this is a BC break, if we're going to do it, it has to happen now obviously.

I'd almost expect it to return the "primary" error object, with any additional errors available in an extra sub-key

An example use-case for where multiple errors are returned would significantly help in deciding if we do need to retain this though.

[rmccue]

Return only a single error

[rmccue]

Replying to dd32:

I'd almost expect it to return the "primary" error object, with any additional errors available in an extra sub-key

Agreed, it's a bit non-obvious there.

An example use-case for where multiple errors are returned would significantly help in deciding if we do need to retain this though.

There's a few potential cases for this. One of the key ones might be multiple validation errors, so e.g. you're trying to create a post, but a) you're not authorised, and b) you sent an invalid post date.

Attached 34551.diff​ as an initial patch at this.

[rmccue]

In 35653:

REST API: Optimise for singular error instances.

Previously, the API returned a list of errors, as WP_Error can hold multiple
error codes internally. This isn't a particularly common use case, and it
makes handling errors on the client side more complex than it needs to be.

Fixes #34551.

[rmccue]

In 35654:

REST API: Update tests for [35653]

See #34551.

[danielbachhuber]

WP_REST_Response::as_error() needs an update.

[danielbachhuber]

Handle new error format in WP_REST_Response::as_error()

[johnbillion]

In 35671:

Update WP_REST_Response::as_error() to handle the new format error responses introduced in [35653].

Props danielbachhuber
Fixes #34551