fix(transmission-done): check_disk_space gives misleading error when df fails

[smartwatermelon]

Problem

In transmission-done.sh, check_disk_space() (line ~489) doesn't check the exit code of df before parsing output. If the df command fails (e.g., NFS mount becomes unavailable), the script exits with a misleading "Insufficient space" error instead of indicating the real cause.

Source

Flagged by sentry[bot] on PR #108 (LOW severity). Pre-existing behavior imported from transmission-filebot.

Suggested fix

Capture df exit code before parsing. If df fails, exit with an error indicating the path may be inaccessible rather than reporting insufficient space.