Improve transaction exceptions

When TXNCXX-223 or TXNCXX-219 are implemented, we will have access to more information on the cause of the exceptions coming back from transactions, both for an operation and the entire transaction.   Lets fold that into our CouchbaseException so we have a nested cause or similar.  Really, most people don't look at this programmatically, but they do print them out, and having this info in their logs will be helpful.   Of course, if they have enabled the client logging they will have this info as well, but still important to have this available no matter what.

NOTE: the TXNCXX-223 is there for me to ponder aligning the transactions with the rest of the client around error_code – no exceptions at all.   That is a bigger (and probably better)way to go, but TXNCXX-219 is just about surfacing the underlying causation.  Ultimately, 223 will depend on figuring out how to nest 2 error_codes cleanly.   Till then, we can do 219, then support it here.