rzv CLI 0.3.1: clearer authorization errors, exact balances

rzv CLI 0.3.1 explains GitHub authorization failures and shows Billing balances exactly.

• rzv github authorizeNames what went wrong with GitHub device authorization, such as an expired code or a failed GitHub request, instead of a generic error.
• rzv billingShows allowance and balance in exact equivalent minutes, including fractions of a minute.
• Release notes are on GitHub at Runzivo/rzv-cli.