Previously, only the first error message for each parameter was made available. Now, all error messages for a parameter are concatenated. Additionally, the detailed error for each parameter is made available in a new `details` section of the validation error. Each error is formatted following the standard REST API error formatting.
The `WP_REST_Server::error_to_response` method has been abstracted out into a standalone function `rest_convert_error_to_response` to allow for reuse by `WP_REST_Request`. The formatted errors now also contain an `additional_data` property which contains the additional error data provided by `WP_Error::get_all_error_data`.
Props dlh, xkon, TimothyBlynJacobs.
Fixes#46191.
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