Impala shell should not try to reconnect when quitting

When the impala-shell is not connected, it will try to reconnect when executing a new command (in precmd).

https://github.com/apache/impala/blob/master/shell/impala_shell.py#L600-L603

if not self.imp_client.is_connected():
   print_to_stderr("Connection lost, reconnecting...")
   self._connect()
   self._validate_database(immediately=True)

This doesn't make sense if the user is quitting the shell (by typing 'quit', 'exit', or hitting Ctrl-D). It should skip the reconnect in this case.