Confusing prompt behavior in arti hsc/hss

When arti hsc {rotate-key, remove-key} prompts you for confirmation, if you type "y" instead of "yes", it will silently wait for you to type "yes", without informing you that the "y" was not good enough. We should make the prompt behavior less confusing (e.g. by aborting if the user types something other than "y" or "n").

Spotted in !3102 (comment 3229811)