Combine Validation & Sanitization callbacks #2695

Opening following a discussion in #core-restapi
#2397 and #2361 (addressing the same issue) are blocked by open questions around sanitization & validation of request parameters. The order in which these functions should be run is not obvious; it has been proposed that they be combined (that Validation behavior be added to the Sanitization callback). From @rmccue :

Should we deprecate having both validation and sanitization? The order is somewhat undefined (and you may want multiple of each in a user-defined order). This would require adding WP_Error support to the sanitization callback.

Adding to the agenda for 9/19 team meeting