Meaning and mitigation of: "WARNING: Failed to open x / y files"

I saw the following output:

```
% mkdir /tmp/new-cache
% sudo btrbk --override 'cache_dir=/tmp/new-cache' extents /media/ssd/btrbk/home-var/*
WARNING: Failed to open 805329 / 805335 files
WARNING: Failed to open 805325 / 805331 files
WARNING: Failed to open 807545 / 807551 files
WARNING: Failed to open 809582 / 809588 files
WARNING: Failed to open 809457 / 809463 files
WARNING: Failed to open 810760 / 810766 files
WARNING: Failed to open 2640077 / 2640085 files
WARNING: Failed to open 3300898 / 3300906 files
TOTAL EXCLUSIVE SUBVOL
333.72 GiB 332.47 GiB /media/ssd/btrbk/home-var/home-var.20250206T1903
350.71 GiB 76.31 GiB /media/ssd/btrbk/home-var/home-var.20250314T1600
277.82 GiB 271.52 MiB /media/ssd/btrbk/home-var/home-var.20250422T1351
277.85 GiB 2.54 MiB /media/ssd/btrbk/home-var/home-var.20250424T1045
277.85 GiB 3.39 MiB /media/ssd/btrbk/home-var/home-var.20250424T1113
277.85 GiB 33.18 MiB /media/ssd/btrbk/home-var/home-var.20250424T1833
277.85 GiB 2.89 MiB /media/ssd/btrbk/home-var/home-var.20250425T1203
277.85 GiB 3.91 MiB /media/ssd/btrbk/home-var/home-var.20250425T1503

Union (8 subvolumes): 687.37 GiB

```

Questions:

- What circumstances would cause files to not be able to be opened by root? (`btrfs scrub` is clean)
- How would I fix things so that the warnings no longer appear?
- Could the UUID be listed against each warning? (eg to be able to delete the cache after fixing it)

Search the codebase for the exact "WARNING: Failed to open" message, then reproduce the reported btrbk extents command with the cache override. Determine and document the causes, remediation, and whether the warning can include a UUID; done means the behavior and cleanup implications are clearly explained.

Written by the indexing model from the issue text.