TUN-3811: Better error reporting on http2 connection termination. Registration errors from control loop are now propagated out of the connection server code. Unified error handling between h2mux and http2 connections so we log and retry errors the same way, regardless of underlying transport.

connection/errors.go

@@ -41,13 +41,13 @@ func serverRegistrationErrorFromRPC(err error) ServerRegisterTunnelError {
 	}
 }
 
-type MuxerShutdownError struct{}
+type muxerShutdownError struct{}
 
-func (e MuxerShutdownError) Error() string {
+func (e muxerShutdownError) Error() string {
 	return "muxer shutdown"
 }
 
-var errMuxerStopped = MuxerShutdownError{}
+var errMuxerStopped = muxerShutdownError{}
 
 func isHandshakeErrRecoverable(err error, connIndex uint8, observer *Observer) bool {
 	log := observer.log.With().