ISPN-4706 Don't abort Marshalling on interruption

The issue was identified via stress tests; I would hit it regularly and everything works much smoother with this fix.
The StressTest itself is not ready for merging yet.

 - https://issues.jboss.org/browse/ISPN-4706

@danberindei I agree on needing to check for starting/stopping caches but I don't see why this is the right place to do so: the writers.get(clazz) method returns null for any JDK type, for example when serializing a String.. su _null_ is nothing special here, it was just preventing remote nodes to retrieve the correct error message about the node actually shutting down!

If you think this was a good place to check for shut down, then the condition check would have been on entering of the method, not just for null values since these are as valid as any other entry.