lib/loopdev: Set errno in is_loopdev on error

lib/loopdev: Set errno in is_loopdev on error

The function is_loopdev does not set errno if the supplied string does
not reference a valid loop device. Fix this to avoid an error message
like this one:

  losetup: /: failed to use device: Success

I prefer this one:

  losetup: /: failed to use device: No such device

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>