Removing the vault's current unlocker left current-unlocker naming it,
so every command that opened the vault failed although another
unlocker remained, and the error pointed at the mnemonic.
RemoveUnlocker now makes the first other unlocker, in the order
`secret unlocker list` shows, current before the removal and returns
its ID, which `secret unlocker remove` prints; removing the last
unlocker also removes current-unlocker. When current-unlocker names an
unlocker that does not exist and the vault has others, the error wraps
ErrUnlockerNotFound and says to run `secret unlocker select`, without
the mnemonic advice.
Unverified: no test covers that advice for a vault other than the
current one, which gets `secret vault select` first.
Model: opus-5-5
