Specifies the fallback behavior (whether the message is allowed or denied) if the handler
does not return a valid response, encounters an error, or times out. (For the timeout period,
Quotas.) If allowed, the message is delivered with returned content to all users
connected to the room. If denied, the message is not delivered to any user. Default:
Identifier of the message review handler. Currently this must be an ARN of a lambda function.
Configuration information for optional message review.