Improve login/auth messages from Arcanist toolset workflows

Summary:
See PHI1802. After D21384, "arc land" and similar with no credentials now properly raise a useful exception, but it isn't formatted readably.

Update the display code to make it look prettier.

Test Plan: Ran "arc land" with no and invalid credentials, got properly formatted output.

Differential Revision: https://secure.phabricator.com/D21387