Enable sanitize_callback to return WP_Error

[websupporter]

As discussed in Slack, it might be helpful, if the sanitization callback could return a WP_Error object like the validation callback.

​https://wordpress.slack.com/archives/core-restapi/p1470200720000471

This patch wants to get this done.

With this patch, we would give developers the opportunity to reject incoming data without using the validation callback. It also enables us to do sanitization and validation in one function in instances where this could be useful.

I've oriented the solution on how it is done in the validation callback:
So, first all Errors are collected and then they will be returned. So if a request has more than one error, you get all the messages at once.

[rmccue]

Updated patch with tests

[rmccue]

Updated the patch and added a unit test to check it's working as expected.

[joehoyle]

In 38601:

REST API: Enable sanitize_callback to return WP_Error.

Give developers the opportunity to reject incoming data without using the validation callback. It also enables us to do sanitization and validation in one function in instances where this could be useful.

Props websupporter, rmccue.
Fixes #37560.