如何捕获org.apache.http.client的异常。当目标机器无法到达时，HttpClient ?

I am running a http connection (using HttpClient.execute) in a asyntask.doInBackgroud. When the http server is not up, I expect my android app prompt the user with the proper information something like "please try again later." But however I try to caught the exception, my app will crash directly. There is no chance to prompt the user about the situation.I am running a http connection (using HttpClien