Make failed files log machine readable (JSON or simply CSV)

[mguentner]

Add output from

nixipfs-scripts/nixipfs/src/mirror_tarballs.py

Line 138 in 6e35344

except subprocess.TimeoutExpired:

to DownloadFailed
then add this to

nixipfs-scripts/nixipfs/src/mirror_tarballs.py

Line 119 in 6e35344

append_failed_entry(work)

finally dump this as JSON to

nixipfs-scripts/nixipfs/src/mirror_tarballs.py

Line 209 in 6e35344

log = "########################\n"