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
When os.UserConfigDir() fails, DetermineStateDir falls back to
os.UserHomeDir(). Previously the error from UserHomeDir was discarded,
which could result in a dangerous root-relative path (/.config/...) if
both calls fail.
Now DetermineStateDir returns (string, error) and propagates failures
from both UserConfigDir and UserHomeDir.
Closes#14