[kvinwang]

Four places swallowed a failure into a default without a trace (#541: silent skips hide corruption). Behaviour is unchanged; each now logs a warning:

• dstack-volume: modprobe dm-verity and udevadm settle results were discarded with let _.
• dstack-volume: a failed blkid fs-type probe became "", which silently drops ro,noload for ext4 volumes.
• dstack-util: an unreadable or malformed gateway cache read the same as a missing one.
• app-compose.sh: jq ... || echo "" made a malformed .sys-config.json look like "no PCCS configured":

$ printf '%s' '{"pccs_url": "https://pccs.example/' > sc.json
$ CFG=$([ -f sc.json ] && jq -r '.pccs_url//""' sc.json || echo ""); echo "exit=$? [$CFG]"
exit=0 []

Verification: cargo test -p dstack-util -p dstack-volume, clippy and fmt clean; bash -n app-compose.sh; ran the new app-compose.sh snippet against a truncated sys-config (warns, continues with empty PCCS_URL).