Users: Merge two similar error messages about usernames with invalid characters.

Props ramiy, garrett-eclipse, chetan200891.
Fixes #47003.

git-svn-id: https://develop.svn.wordpress.org/trunk@48195 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

@@ -1245,7 +1245,7 @@ class WP_REST_Users_Controller extends WP_REST_Controller {
 		if ( ! validate_username( $username ) ) {
 			return new WP_Error(
 				'rest_user_invalid_username',
-				__( 'Username contains invalid characters.' ),
+				__( 'This username is invalid because it uses illegal characters. Please enter a valid username.' ),
 				array( 'status' => 400 )
 			);
 		}