Paying a non-existent account yields T00

[sappenin]

Paying a PaymentPointer that doesn't exist responds with a T00 error, but instead should stop the payment.

This is primarily a problem in StreamReceiver likely. A payment to a non-existent account should be a F error, which would terminate the SimpleStream sender appropriately.

[sappenin]

StreamSender is now deprecated, but this needs to be validated in ILP Pay.

[sappenin]

StreamSender is deprecated, but this is worth verifying in StreamPayer.