A metacache listing that misses quorum purely because the volume or path
is absent on a quorum of drives (every drive reports VolumeNotFound or
FileNotFound) is a benign, expected outcome: list_path_raw already returns
VolumeNotFound/FileNotFound and lets the caller decide how to react.
The common trigger is a startup race where the system bucket (.rustfs.sys)
is not yet created on every drive when an early reader such as the IAM
config loader lists config/iam/. Logging that at error prints a scary
message during normal boot and, in #5076, misled a user into blaming it
for unrelated upload failures.
Add is_benign_not_found_listing_failure() and demote the pure not-found
case to debug (state = "quorum_not_found"), keeping error for listings
that failed for a real reason (I/O, timeout, corruption). Return value and
control flow are unchanged. Add a unit test for the classifier.
Refs: #5076
