Don't print startup warnings error when --auth enabled

The startup warning will never be able to be printed until the user is authenticated, so printing an error message on every shell startup will only generate user confusion. We should swallow the error message.

ubuntu@m110:~$ mongo m110.realm3.10gen.me

MongoDB shell version: 2.4.0-rc0

connecting to: m110.realm3.10gen.me/test

Error while trying to show server startup warnings: unauthorized