We should use ```logger.exception``` instead of print_exc.

But in this case it isn't needed at all, the callstack should be printed in the ```_call_exception_handlers``` method by line ```logger.warning("keep running regardless of exception [%s]", exception)```.

This ```logger.warning``` statement should be renamed to ```logger.exception``` to get the callstack.  This has already been done on the Bartercast3 branch which should be merged into devel next week.