libfstools: Print error in case of loop blkdev failure

It took me quite some time today(while fixing squashfs+overlay on
armvirt) to find out, that I was missing support for loop block device
in kernel, so I'm adding error message which might be helpful for
someone else in the future as well.

Acked-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Petr Štetiar <ynezz@true.cz>