Based on policy, either raise defect or call register_defect.
handle_defect(obj, defect)
defect should be a Defect subclass, but in any case must be an
Exception subclass. obj is the object on which the defect should be
registered if it is not raised. If the raise_on_defect is True, the
defect is raised as an error, otherwise the object and the defect are
passed to register_defect.
This method is intended to be called by parsers that discover defects.
The email package parsers always call it with Defect instances.