TransactionExceptions need to be surfaced properly

Currently I just raise a generic CouchbaseException. Ideally, we'd have the TransactionOpFailed and TransactionException use CouchbaseException as a base class, and have all the necessary fields in them. In the case of the TransactionException, which is raised when a transaction fails (rather than just an operation failing), we really need derived classes for the failure types (FAIL, EXPIRY, COMMIT_AMBIGUOUS).