The error message when the second image is not available is incorrect,
replace "first image" with "second image".

Fixes CoverityScan CID#1077508 ("Copy-paste error")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>