Some exceptions make XStream go nuts

We need a way to make sure that some logged exception won't make XStream go nuts and crash the farm like it happen sometime.

Some exceptions can be pretty weird (just look at XWikiException in XWiki 1.0...) but I guess the safest is to introduce a custom serializer for anything extending Throwable and serialize only the Throwable fields (message, cause, etc). If the unserialize custom Exception is missing some information then so be it (we could probably skip the special serializer for any Exception with @Serializable annotation).