Cache class name lookup failure

This patch improves the performance of CachingMapper by having it remember failed class lookup.

Imagine we are loading a large number of XML files via XStream that refers to one class that doesn't exist any more. My converter gracefully recovers from it by ignoring just the portion that uses the non-existent class, but in the current XStream code, this greatly suffers performance-wise by repeatedly trying to load a non-existent class (which results in a lookup across classloader hierarchy.)