Provide more helpful feedback than just "Cheatin' uh?" for permission errors in wp-admin/network/site-users.php.

props ericlewis, kraftbj, lukecarbis, mrmist.
fixes #33683. see #14530.
Built from https://develop.svn.wordpress.org/trunk@33888


git-svn-id: http://core.svn.wordpress.org/trunk@33857 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/network/site-users.php

@@ -136,8 +136,13 @@ if ( $action ) {
 					$user_id = (int) $user_id;
 
 					// If the user doesn't already belong to the blog, bail.
-					if ( !is_user_member_of_blog( $user_id ) )
-						wp_die( __( 'Cheatin’ uh?' ), 403 );
+					if ( ! is_user_member_of_blog( $user_id ) ) {
+						wp_die(
+							'<h1>' . __( 'Cheatin&#8217; uh?' ) . '</h1>' .
+							'<p>' . __( 'One of the selected users is not a member of this site.' ) . '</p>',
+							403
+						);
+					}
 
 					$user = get_userdata( $user_id );
 					$user->set_role( $_REQUEST['new_role'] );