Check if a external drive is mounted before snapshot / backup

Hello there!

I had this happen to me before and is happening again. I have mine set up to make local snapshots and also backup and snapshots to a external drive.
Some times BTRBK ignores that the drive is not mounted and backups/snapshots anyway. Did I configure something wrong?

I use /btfs_pool for the snapshots /btrfs_backup for the external backup + snapshots drive. The drive is not connected right now, but it also happened many times.

```
ls /btrfs_backup/ ✘ 16 10:19:03
home.20221005T0001/ home.20221026T102727-0300/ home.20221030T000000-0300/ home.20221105T000143-0300/ root.20221008T1439/ root.20221217T000000-0300/ root.20221221T000000-0300/
home.20221009T081430-0300/ home.20221027T081940-0300/ home.20221031T000001-0300/ home.20221106T000000-0300/ root.20221106T000000-0300/ root.20221218T000002-0300/ root.20221222T000000-0300/
home.20221016T000102-0300/ home.20221028T000001-0300/ home.20221101T000000-0300/ home.20221107T092408-0300/ root.20221116T000000-0300/ root.20221219T000000-0300/ root.20221223T000000-0300/
home.20221023T083304-0300/ home.20221029T101632-0300/ home.20221102T000000-0300/ home.20221108T000000-0300/ root.20221216T084715-0300/ root.20221220T000001-0300/ root.20221224T000000-0300/
```

```
/etc/fstab
...
# BTRBK backups
UUID=3f835c73-1a8c-4bca-9646-4cbd60bc7c6d /btrfs_pool btrfs subvolid=5,compress=zstd:1,noatime,x-systemd.device-timeout=0,x-systemd.after=/ 0 0
UUID=/dev/mapper/luks-afaa1238-7ece-40e4-a069-c33b3ccf9140 /btrfs_backup btrfs subvolid=5,nofail,space_cache=v2,compress=zstd:3,noatime,x-gvfs-show,x-systemd.device-timeout=0,x-systemd.after=/ 0 0
...
~ ❯ doas btrbk list config
SOURCE_SUBVOLUME SNAPSHOT_PATH SNAPSHOT_NAME TARGET_PATH
/btrfs_pool/root /btrfs_pool/btrbk_snapshots root /btrfs_backup
/btrfs_pool/home /btrfs_pool/btrbk_snapshots home /btrfs_backup

~ ❯ doas btrbk list source
SOURCE_SUBVOLUME SNAPSHOT_PATH SNAPSHOT_NAME
/btrfs_pool/root /btrfs_pool/btrbk_snapshots root
/btrfs_pool/home /btrfs_pool/btrbk_snapshots home

~ ❯ cat /etc/btrbk/btrbk.conf
transaction_log /var/log/btrbk.log
lockfile /var/lock/btrbk.lock
timestamp_format long-iso
stream_buffer 256m
#incremental strict

snapshot_dir btrbk_snapshots
snapshot_preserve_min latest
snapshot_preserve 3h 5d 0w 0m 0y
target_preserve_min latest
target_preserve 0h 14d 6w 4m 1y
archive_preserve 0h 1d 1w 1m 1y
archive_preserve_min latest

volume /btrfs_pool
snapshot_create ondemand
target send-receive /btrfs_backup
subvolume root
subvolume home

# Enable stream buffer. Adding a buffer between the sending and
# receiving side is generally a good idea.
# NOTE: If enabled, make sure to install the "mbuffer" package!
#stream_buffer 256m
#btrfs_commit_delete yes

#snapshot_dir btrbk_snapshots
#incremental yes
#snapshot_preserve_min 18h
#snapshot_preserve 48h 7d
#target_preserve_min 6h
#target_preserve 24h 24d 24w

#volume /btrfs_pool
# snapshot_create ondemand
# incremental yes
# target send-receive /btrfs_backup
# subvolume root
# subvolume home

~ ❯ cat /usr/lib/systemd/system/btrbk.service
[Unit]
Description=btrbk backup
Documentation=man:btrbk(1)

[Service]
Type=oneshot
ExecStart=/usr/bin/btrbk run

~ ❯ cat /usr/lib/systemd/system/btrbk.timer
[Unit]
Description=btrbk daily backup

[Timer]
OnCalendar=daily
AccuracySec=10min
Persistent=true

[Install]
WantedBy=timers.target

```

Start with the reported /etc/btrbk/btrbk.conf and the btrbk run entry point invoked by /usr/bin/btrbk, then review the systemd service and timer files. Reproduce a scheduled run with /btrfs_backup unmounted and compare the resulting snapshots, backups, and log output with the expected safe behavior.

Written by the indexing model from the issue text.