rework CacheWriter writeAll() and deleteAll() to permit partial success

you're up late. I still might do something in this method but I don't know why it has to be a factor in this PR. 

the thought that troubled me earlier tonight was turning one exception (BulkCacheWriterException) into some other type related exception and the value in that. 

Anyway, I can't bless my own PR can I? :-) 

(like I've never merged my own stuff)