It would be better to raise an exception* upon receiving a cookie. On the other hand, I presume cookies are stored in files that any process can mess with, so reading failures are always a possibility. So if you want to catch a (very rare) failure, to do something useful, then you should do so.

*My preference versus silent rejection. But I do not do web programming.

Unless the current doc says something I missed, I think either change from the current 'store anything' policy would be a feature request. This is assuming that current behavior is unchanged from 2.5.