mon: Abort device health when device not found

mon: Abort device health when device not found

If `store->get_devname()` returns an empty device name, it means it couldn't
determine the device that backs the monitor DB store directory.

This can happen if `ceph-mon` runs with `PrivateDevices=yes` in systemd, or
within a container where the host `/dev` isn't exposed.

This commit makes sure we abort trying to get the device health at that point,
and return an appropriate error.

Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>