04272613a7
The CANCELLED return value from an operator is intended for signaling that the operator aborted execution without changing anything, and an Undo stack entry should not be created. When a Python operator crashes, it is not safe to assume that it did nothing, so it should interpret it as FINISHED instead. Otherwise, the undo system behavior after an operator failure feels broken. Differential Revision: https://developer.blender.org/D6241