wiping: log_warn instead of log_error if blkid wipe ignored for a signature

wiping: log_warn instead of log_error if blkid wipe ignored for a signature

Comply with the rules we have for log_error and log_warn...

$ pvcreate /dev/sda1
  Failed to get offset of the xfs_external_log signature on /dev/sda1.
  1 existing signature left on the device.
  Aborting pvcreate on /dev/sda1.

$ pvcreate /dev/sda1 --force
  WARNING: Failed to get offset of the xfs_external_log signature on /dev/sda1.
  Physical volume "/dev/sda1" successfully created