Clarification on snapshot_create ondemand for unplugged disks

hello, I'm having some trouble setting this up to take snapshots locally with a copy to USB disk that may or may not be connected, as per your example on the readme page, config:

```
snapshot_preserve_min 2d
snapshot_preserve 14d

target_preserve_min no
target_preserve 20d 10w *m

snapshot_dir @snapshots
snapshot_create ondemand

volume /mnt/btr_pool
target /mnt/backup/@backups
subvolume @
subvolume @home
```
when USB disk is detached, /mnt/backup exists, but `@backups` (a subvolume) does not. btrbk complains however:

```
WARNING: Skipping target "/mnt/backup/@backups": Failed to fetch subvolume detail
WARNING: ... Command execution failed (exitcode=1)
WARNING: ... sh: readlink -v -e /mnt/backup/@backups
WARNING: ... readlink: /mnt/backup/@backups: No such file or directory
WARNING: Skipping target "/mnt/backup/@backups": Failed to fetch subvolume detail
WARNING: Skipping cleanup of snapshots for subvolume "/mnt/btr_pool/@", as at least one target aborted earlier
WARNING: Skipping cleanup of snapshots for subvolume "/mnt/btr_pool/@home", as at least one target aborted earlier
--------------------------------------------------------------------------------
Backup Summary (btrbk command line client, version 0.29.2-dev)

Date: Fri Jul 10 15:35:26 2020
Config: /etc/btrbk.conf

Legend:
=== up-to-date subvolume (source snapshot)
+++ created subvolume (source snapshot)
--- deleted subvolume
*** received subvolume (non-incremental)
>>> received subvolume (incremental)
--------------------------------------------------------------------------------
/mnt/btr_pool/@
!!! Target "/mnt/backup/@backups" aborted: Failed to fetch subvolume detail

/mnt/btr_pool/@home
!!! Target "/mnt/backup/@backups" aborted: Failed to fetch subvolume detail

NOTE: Some errors occurred, which may result in missing backups!
Please check warning and error messages above.
```

is this to be expected and I can ignore the error?