Handle reporting non-existing posts.

inc/config.php

@@ -1187,6 +1187,7 @@
 	$config['error']['fileext']		= _('Unsupported image format.');
 	$config['error']['noboard']		= _('Invalid board!');
 	$config['error']['nonexistant']		= _('Thread specified does not exist.');
+	$config['error']['nopost']		= _('Post specified does not exist.');
 	$config['error']['locked']		= _('Thread locked. You may not reply at this time.');
 	$config['error']['reply_hard_limit']	= _('Thread has reached its maximum reply limit.');
 	$config['error']['image_hard_limit']	= _('Thread has reached its maximum image limit.');