catch-all handler for messages with no receivers

[mjhm]

If a web service makes send an exocom message which never returns due to an error somewhere in a chain of services, then the web service just hangs. Ideally for such unexpected errors we would want to return a 500 status error. The service that errors can send a generic error message, but I don't see a way for the web service to capture that error, without passing it back through the service chain.

It's possible for services to generate messages (e.g. for errors) that aren't explicitly handled by any service. Then exocom prints out something like

Warning: No receivers for message 'show error' registered

Ideally we'd also want to be able to log the error along with it's payload so that the problem can be diagnosed.

[charlierudolph]

We could add a timeout when waiting for a message reply which would alert the sender that there either there were no listeners or they are unable to process the message in a timely manner. This is implemented in the go version of exorelay already.

Updating the log to show the payload is pretty easy.

[mjhm]

Hmm -- timeouts are rather inelegant, non-specific, and could yield false errors for slow operations. That may be the best we can reasonably do though.

In my unreasonable ideal world I think I'd want to register a short lived message receiver in the web-service that only receives error messages that were specific to the original request.

[charlierudolph]

Hmm okay we could build something into exocom so that on a message that no one is listening to it sends back message no-receivers with the same activity id which would be heard as a reply