accounts: say when ATGC_ACCOUNT names an account that isn't logged in #1350

Two account-name edge cases:

• ATGC_ACCOUNT=nobody-here.test.pds.did.bot atgc pr list succeeds silently, without saying that account isn't logged in.
• A single-label name gets an example using a real person's DID, instead of listing the logged-in accounts the way the unknown-handle error does.