Better onboarding error message

[jblachly]

If a config dir doesn't exist yet (because generate-auth-token hasn't been run), the error message is a bit opaque:

james@ubuntu:~/src/chamberlain$ cargo run --bin chamberlain -- -a http://localhost:22290/ get-info
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.40s
     Running `target/debug/chamberlain -a 'http://192.168.1.44:22290/' get-info`
Error: Os { code: 2, kind: NotFound, message: "No such file or directory" }

Would be nice to suggest to the user that they run generate-auth-token