Lockfiles with invalid JSON content caused bisync to fail permanently
because lockFileIsExpired() logged the decode error but still fell
through to the "valid lock file" path with zero-value TimeExpires.
Now when a JSON decode error is detected:
- If --max-lock is set (< basicallyforever): treat garbled lockfile as
expired, mark listings failed, and proceed (safe assumption: the
previous bisync run crashed and left garbage).
- If --max-lock is not set (default): log a clear error telling the
user the lockfile needs manual inspection, and return false.
