Warn when running in signed-out mode

[hex539]

The contest downloader has to do a lot of guesswork when it's run without credentials. It can mostly work things out, but some things like WA/RTE/TLE distinction are msising and there may be other bugs as it doesn't get a lot of testing in reality.

We should warn if a user runs the tool like this, just in case they have credentials available to plug in.