DDL crashes on legitimate error from SessionManager::newTxnID()

Looking into this problem, Dave Hill & I found a couple things that need improvement.

https://support.mariadb.com/view.php?id=24999

1) DDL shouldn't crash when it gets an error from newTxnID(), it should return a sensible error message to the user.
2) The controllernode should log the error it gets from SessionManagerServer so we have a better idea of what's going on. In this case it might not have been possible (I suspect the problem was filesystem corruption and/or a driver problem and/or a controller problem) but the code should at least try.