[REEF-76] Throw a ConnectException after failure of all the connection retries in NettyMessagingTransport.open

`NettyMessagingTransport.open(...)` throws a `ConnectException` after failure of all the connection retries 


JIRA: [REEF-76]: https://issues.apache.org/jira/browse/REEF-76

I'm not entirely sure I follow the original control flow of this method. But with this change, none of the code below this block seems to matter anymore, as there really are only two cases now:

  * We found a port that works, in which case we will have `return`ed out of the loop above.
  * We did not find one, in which case we exit the loop matching this if block's condition and we throw an exception. The condition seems to be redundant in that case.

What purpose does / did the remainder of this method serve? 

@bgchun Can you comment on this?