du_bytes returned None uniformly for every du failure, and both
du_bytes and measure_agent_disk had no logging at all, so a container
rootfs du couldn't read (permission, I/O error, unparseable output)
silently reported as 0 disk with no signal in the journal.
Distinguish the expected case — the path plain doesn't exist, e.g. a
destroyed-but-kept agent's rootfs or a not-yet-created state dir — from
an actual read failure, and warn only on the latter, with the path plus
whichever of the failure (spawn error, exit status, stderr, unparseable
stdout) applies.
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