Preloading does not report unmarshalling errors

When cache preloading hits an unmarshalling error, it stops in order to preserve the consistency of the cache. However, the thrown exception and the log won't contain the key with the problem, just a generic Unable to preload! message.

We should at the very least include the key in the exception message. But we could also provide a configuration option to ignore unmarshalling errors and only log a summary with the keys that could not be loaded at the end.