Make base abstract methods raise NotImplementedError
Make base abstract methods raise NotImplementedError

edx/XBlock,nagyistoce/edx-XBlock,edx-solutions/XBlock,nagyistoce/edx-XBlock,EDUlib/XBlock,4eek/XBlock,lovehhf/XBlock,edx-solutions/XBlock,edx/XBlock,mitodl/XBlock,EDUlib/XBlock,lovehhf/XBlock,mitodl/XBlock,4eek/XBlock