agent: surface websocket errors in logs

The websocket interface used for `alloc exec` has to silently drop client send
errors because otherwise those errors would interleave with the streamed
output. But we may be able to surface errors that cause terminated websockets
a little better in the HTTP server logs.

command/agent/alloc_endpoint.go

@@ -556,10 +556,14 @@ func (s *HTTPServer) execStreamImpl(ws *websocket.Conn, args *cstructs.AllocExec
 	handler(handlerPipe)
 	// stop streaming background goroutines for streaming - but not websocket activity
 	cancel()
-	// retreieve any error and/or wait until goroutine stop and close errCh connection before
+	// retrieve any error and/or wait until goroutine stop and close errCh connection before
 	// closing websocket connection
 	codedErr := <-errCh
 
+	// we won't return an error on ws close, but at least make it available in
+	// the logs so we can trace spurious disconnects
+	s.logger.Debug("alloc exec channel closed with error", "error", codedErr)
+
 	if isClosedError(codedErr) {
 		codedErr = nil
 	} else if codedErr != nil {