schedule/sendExisting - Sends an existing schedule.

Method parameters:
id - The ID of the schedule to send.
receiver_username - The Zvini username of the receiver.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
SCHEDULE_NOT_FOUND - A schedule with the ID doesn't exist.
ENTER_RECEIVER_USERNAME - The receiver username is empty.
INVALID_RECEIVER_USERNAME - The receiver username is invalid.
RECEIVER_NOT_FOUND - No such receiver with the username.
RECEIVER_NOT_RECEIVING - The receiver hasn't opened a connection to receive schedules from you.