Fixed reraising of plotting exceptions #1158
This has been plaguing me for a long time. In python2 I always get these exceptions when there's an issue with the options. I think this is due to the nested try/excepts. Reraising an exception in this way differs slightly between py2 and py3 hence extra conditional.
I've always found reports of this error very mysterious - not only does the error message make no sense (all our classes derive from object, we don't have any old style classes!) but I've never been able to reproduce it myself.