Damien, thanks for catching this.

The change was not intended.

There are two kind of exceptions raised by send.
1. Where a pre-condition is not met, e.g. a generator is already ruuning (caller errors)
2. When the generator/coroutine raises an exception (callee exceptions).

(1) does not change the state of the generator, (2) does.

Sending non-None to a generator that has not started falls into kind 1, IMO. So we should revert the change.