Return a 404 error if the request feed type doesn't exist not a 500. Fixes #12341 props aaroncampbell.

git-svn-id: https://develop.svn.wordpress.org/trunk@13408 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/functions.php

@@ -1706,7 +1706,7 @@ function do_feed() {
 	$hook = 'do_feed_' . $feed;
 	if ( !has_action($hook) ) {
 		$message = sprintf( __( 'ERROR: %s is not a valid feed template.' ), esc_html($feed));
-		wp_die($message);
+		wp_die( $message, '', array( 'response' => 404 ) );
 	}
 
 	do_action( $hook, $wp_query->is_comment_feed );