Update huburi for FeedSub if PuSH signature is invalid

This because some remote server might have used third party PuSH hubs
but switch and we don't know about it.

Possible risks here are of course MITM that could force us to rediscover
PuSH hubs from a feed they control, but that currently feels ... meh.

plugins/OStatus/lib/feeddbexception.php

@@ -0,0 +1,12 @@
+<?php
+
+class FeedDBException extends FeedSubException
+{
+    public $obj;
+
+    function __construct($obj)
+    {
+        parent::__construct('Database insert failure');
+        $this->obj = $obj;
+    }
+}