Reworked File->getUrl to throw exception

In case you require a local URL and one can't be generated, throw
FileNotStoredLocallyException(File $file)

lib/filenotstoredlocallyexception.php

@@ -0,0 +1,15 @@
+<?php
+
+if (!defined('GNUSOCIAL')) { exit(1); }
+
+class FileNotStoredLocallyException extends ServerException
+{
+    public $file = null;
+
+    public function __construct(File $file)
+    {
+        $this->file = $file;
+        common_debug('Requested local URL for a file that is not stored locally with id=='._ve($this->file->getID()));
+        parent::__construct(_('Requested local URL for a file that is not stored locally.'));
+    }
+}