CLI - Tell users if they're already logged in

[bfops]

Description of Changes

Currently, spacetime login just silently returns if the user is already logged in.

This PR adds a message informing the user that they are already logged in.

API and ABI breaking changes

None

Expected complexity level and risk

1

Testing

• When I run spacetime login, I see the message.

$ cargo run -- login
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.27s
     Running `target/debug/spacetime login`
You are already logged in.
If you want to log out, use spacetime logout.

[cloutiertyler]

Seems reasonable.