Handle Errno::ECONNREFUSED when URL is not a valid connection

[rreinhardt9]

If you supply any URL for the signal_url that is a validly formatted URL but that URL isn't accepting connections (it doesn't exist, for example) it raises Errno::ECONNREFUSED.

A common way to encounter this would be if you set up a URL for an endpoint that doesn't exist yet, or maybe you make a mistake entering the URL.

When this occurs, we don't want to raise this error in the parent app. Rather, let's handle this in a similar way to how timeouts were handled in #9 and note the status on the transmission. This will allow for debugging and triggering other behavior; for example an app might want to deactivate a connection that has over a threshold of this error (hmm, that might make another nice Demux feature too 😆 )