Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 29366)
+++ src/wp-includes/functions.php	(working copy)
@@ -2654,9 +2654,21 @@
 function wp_send_json_error( $data = null ) {
 	$response = array( 'success' => false );

-	if ( isset( $data ) )
-		$response['data'] = $data;
+	if ( isset( $data ) ) {
+		if ( is_wp_error( $data ) ) {
+			$result = array();
+			foreach ( $data->errors as $code => $messages ) {
+				foreach ( $messages as $message ) {
+					$result[] = array( 'code' => $code, 'message' => $message );
+				}
+			}

+			$response['data'] = $result;
+		} else {
+			$response['data'] = $data;
+		}
+	}
+
 	wp_send_json( $response );
 }