When an unlocker directory exists but is missing unlocker-metadata.json,
log a debug warning and skip it instead of returning a hard error that
crashes the entire 'unlocker ls' command.
Closes#1
NumSecrets() previously looked for non-directory, non-'current' files
directly under each secret directory, but the only children are
'current' (file, excluded) and 'versions' (directory, excluded),
so it always returned 0.
Now checks for the existence of the 'current' file, which is the
canonical indicator that a secret exists and has an active version.
This fixes the safety check in UnlockersRemove that was always
allowing removal of the last unlocker.
