move all exception declaration to their own file

lib/AudioBook.php

@@ -2,12 +2,6 @@
 
 require_once('DbMapping.php');
 
-class BookNotFoundException extends WebException {
-    public function __construct($code) {
-        parent::__construct('BookNotFound', "The book with code $code was not found", -404);
-    }
-}
-
 /**
  * AudioBook is mapped on a Notice from NetBiblio
  *