Merge multiple WP_Error objects together.

wp-error-merge.php

<?php
function wpcom_wp_error_merge() {
  $wp_error_merged = new WP_Error();
	$wp_errors = func_get_args();
	
	foreach ( $wp_errors as $wp_error ) {
		if ( ! is_wp_error( $wp_error ) )
			continue;

		foreach ( $wp_error as $key => $errors ) {
			foreach ( $errors as $error ) {
				$wp_error_merged->add( $key, $error );
			}

			if ( isset( $wp_error->error_data[ $key ] ) ) {
				$wp_error_merged->add_data( $wp_error->error_data[ $key ], $key );
			}
		}
	}	
	return $wp_error_merged;
}

@wpscholar, your link doesn't lead anywhere. I guess it should be that one.

For anyone hitting this page, as of WordPress 5.6 WP_Error allows merging objects together. See https://core.trac.wordpress.org/ticket/38777