Expose the failure on the exception the client raises #97

Motivation

The client raises the application's own exception classes with failure.to_s as the message (client.rb:203). An application that needs the failure's code or cause must parse the message. HEY does exactly that to show a lock placeholder for a password-protected PDF: it matches mutool's stderr inside the message (encryption_aware_pdf_previewer.rb:27).

Urgency

Low. Nothing is broken today, because Codes documents the message format. This change matters once a cell sends causes an application acts on, such as the protected cause in #96. Without it, an application still has to parse the message to read that cause.

Implementation suggestion

Donal's carry-the-failure branch (4cdf8fa1) extends each raised exception with a HotCell::Verdict module. The module exposes the HotCell::Failure as hot_cell_failure, so an application reads error.hot_cell_failure.cause. It can also rescue HotCell::Verdict, whatever classes its registration injected. The branch includes tests in hotcell-client/test/classification_test.rb. To finish it, rebase onto master, document the module in docs/client-api.md and docs/codes.md, and add a changelog entry.