dev-ext: issue error if external_device_info_source=udev and udev db record incomplete

dev-ext: issue error if external_device_info_source=udev and udev db record incomplete

Udev db records are marked as not initialized (incomplete) on timeout.
Issue an error message whenever LVM finds such records so users are
aware that something's going wrong with udev db.

This is important in case we use devices/external_device_info_source="udev"
where udev database records are used to do various filtering decisions.

For example:

udev log of timed out worker:

Nov 11 13:02:25 raw.virt systemd-udevd[607]: seq 1997 '/devices/virtual/block/dm-2' is taking a long time
Nov 11 13:04:25 raw.virt systemd-udevd[607]: seq 1997 '/devices/virtual/block/dm-2' killed
Nov 11 13:04:25 raw.virt systemd-udevd[607]: worker [11221] terminated by signal 9 (Killed)
Nov 11 13:04:25 raw.virt systemd-udevd[607]: worker [11221] failed while handling '/devices/virtual/block/dm-2'
...

LVM also issues error message visibly if incomplete udev db record is found,
devices/external_device_info_source="udev" is set:

$ pvs
  Udev database has incomplete information about device /dev/dm-2.
  Failed to get external handle for device /dev/dm-2 [udev].
  ...