improve error message

Signed-off-by: Future-Outlier <[email protected]>

flyteplugins/go/tasks/plugins/webapi/agent/plugin.go

@@ -83,6 +83,10 @@ func (p Plugin) Create(ctx context.Context, taskCtx webapi.TaskExecutionContextR
 	agent := getFinalAgent(taskTemplate.Type, p.cfg, p.agentRegistry)
 
 	client := p.cs.agentClients[agent.Endpoint]
+	if client == nil {
+		return nil, nil, fmt.Errorf("agent:[%v] is not connected, please check if the agent is up and running", agent)
+	}
+
 	finalCtx, cancel := getFinalContext(ctx, "CreateTask", agent)
 	defer cancel()