Actually I should amend that.  This should be except ``(ValueError, <other_exceptions_that_might_be_raised_by_bytes_(cookie))``.  Rationale: it is part of the serializer API that serializers must raise ValueError on malformed inputs.  They should never raise a different error.  If they do. it's a bug in the serializer.  Of course we should probably check that existing serializers don't raise anything but ValueError roo.