Inconsistent error messages on empty pools

[ydahhrk]

NAT64 Jool:

$ jool --pool4 --display
  (empty)

$ jool --pool6 --display
Object not found (System error -12)

SIIT Jool:

$ jool_siit --pool6 --display
Object not found (System error -12)

$ jool_siit --pool6791 --display
Object not found (System error -12)

$ jool_siit --blacklist --display
Object not found (System error -12)

$ jool_siit --eamt --display
  (empty)

(Assuming --pool4 and --eamt don't segfault - see #150.)

This doesn't cause any crying kids - all it is is visually inconsistent. While "(empty)" is the expected output, "Object not found (System error -12)" is not really misleading.

Thanks to Edgar Rodriguez for pointing this out.

[ydahhrk]

Merged; closing.