Message takes a string message that is to be sent in an `OK` or `CLOSED` command and prefixes it with "<prefix>: " if it doesn't already have an acceptable prefix.

T is a relay message sent in response to an EventEnvelope to indicate acceptance (OK is true), rejection and provide a human readable Reason for clients to display to users, with the first word being a machine readable reason type, as listed in the RejectReason* constants above, followed by ": " and a human readable message.