Race condition in cache creation shows an error in the logs

Crowd's caches are created by request, but there's a race condition: we check that they exist and then created them. Simultaneous attempts to create would see one of those attempts fail. We should use addCacheIfAbsent instead so EhCache takes care of the possible race.
The exception is caught and logged, so this shouldn't be causing problems, but fixing it will clear up noise in the logs.