[ZhuchkaTriplesix]

Part of #1761. Stacked on #1794 (base is its branch; GitHub retargets it to dev when #1794 is merged).

• failureMessage(action, error, where:, stack:, driver:) next to reportError: logs the failure and returns <action>: <reason> for the views that show an inline error line instead of a toast.
• The 18 raw '... failed: $e' messages in redis_key_editor, redis_keys_view and redis_connection_form use it with DatabaseDriver.redis; the catch blocks now keep the stack.
• Text for an unrecognised error is unchanged (the error's own text); a recognised driver error now reads as the mapped message with its hint. No test depended on these texts. Not run locally, CI will.