Support returning an error message from Image to Publication

[mjpt777]

When two-way, aka duplex, communication is being established and the return channel is invalid it would be useful to return an error somehow to the initiator. An example would be an invalid control response channel for an archive client. A solution would be if the receiving image from the connection initiation could return an error message to the source publication. The API could look something like the following.

void Image.returnError(String) would send the error back via the control channel used for status messages and NAKs to the sender.

The sender could then apply this error to the source publication and close it. The publisher could detect the unexpected close and then call String Publication.errorMessage() to get the message.

The error message could be restricted to a max length of an MTU payload. To cope with loss the error could be sent on an interval like status messages until acknowledged by the sender or timeout when heartbeats are no longer received.