mongos with --upgrade and missing --auth flag prints "config servers not in sync" message

On a sharded cluster with auth turned on, start up a 2.4 mongos with --upgrade to do the metadata upgrade but forget to include --auth:

[mongosMain] ERROR: config servers not in sync! no config servers reachable

The error is confusing, the config servers are in sync but mongos just can't connect to them because of missing auth params. Should be some kind of auth error message instead.