This ensures NotImplemented is returned on error so that the Python
interpreter will handle the operators correctly instead of throwing an
exception. NotImplemented was not being returned for non-builtin wrappers
when the operator overload did not have a function overload.
See PEP 207 and https://docs.python.org/3/library/constants.html#NotImplemented
Mentioned in SF patch #303 and SF bug #1208.
