[GitHub] [rocketmq] RongtongJin opened a new pull request #1415: [ISSUE 504] Polish 'No route info of this topic' exception

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RongtongJin opened a new pull request #1415: [ISSUE 504] Polish 'No route info of this topic' exception
URL: https://github.com/apache/rocketmq/pull/1415
 
 
   ## What is the purpose of the change
   [ISSUE 504](https://github.com/apache/rocketmq/issues/504).
   
   ”No route info of this topic“ exception is a classic problem for rocketmq beginners.
   
   If no route data of the topic is retrieved from name server, this exception will be thrown. However, in many cases, the lack of routedata in nameserver is not the real cause, such as the following situations：
   
   1. No nameserver address is set.
   2. Network problems (Producer can't connect to nameserver, such as firewall).
   3. Netty version issue.
   
   The real exceptions are written to the log, and 'no route info of this topic' is thrown to console, which mislead the users. 
   
   ## Brief changelog
   
   This PR polish exception of  'no route info of this topic'. It will throw more specific exceptions to users. For example, due to network problems, the route data fails to be obtained, and the wrapped remotingXXXXXException is directly thrown to users and users can locate the problem according to exception chaining.
   
   ## Verifying this change
   
   XXXX
   
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   - [x] Format the pull request title like `[ISSUE #123] Fix UnknownException when host config not exist`. Each commit in the pull request should have a meaningful subject line and body.
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