Set to either the generated message, or to an empty string if a message is unavailable (Iguana will poll again later). Ignored if StopChannel is set - it is not possible to transmit a message and request 'stop channel' simultaneously. Deprecated behavior: if MessageIsError is set, the contents of OutboundMessage are interpreted as an error-level log message. This behavior has been retained for backwards compatibility with existing plugins. New or updated plugins should use the addLogMessage method instead.