raise, rather than raise ex on thrift transport error

Preserves original backtrace.

lib/rb/jp/server/handler.rb

@@ -84,7 +84,7 @@ def rescue_connection_failure
             success = true
           rescue Mongo::ConnectionFailure => ex
             retries += 1
-            raise ex if retries >= @retry_attempts
+            raise if retries >= @retry_attempts
             sleep(@retry_delay)
           end
         end