Improve error messages when not logged in

[vpetersson]

If you run the edge-app command when not logged in, you get this:

$ screenly edge-app run 
thread 'main' panicked at src/cli.rs:822:48:
Failed to load authentication.: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" })
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Granted, it says failed to load authentication, but that is hardly a useful message.