v0.24.0 — Etherscan errors are no longer empty wallets

Fixed

• BSC, Optimism and Base looked empty on a free Etherscan key. Etherscan's
  free plan does not serve those chains and says so in an error, but the
  error passed for "no transactions": every wallet there showed a zero
  balance and no history, and the answer was cached for an hour. Any error
  Etherscan returns is now reported — for these chains with what is needed
  (a paid plan) — and never cached; errors cached by older versions are
  ignored. crypttrace chains and the README say which chains need the paid
  plan.
• A network failure talking to Etherscan is reported as such instead of
  surfacing as a raw requests exception.
• crosschain silently skipped every chain it could not read, so "no
  arrival found" also covered chains it never searched. It now lists them
  under "Not searched".