It's a pathetic error message but it does make sense I think. The computar tried to boot a disk but it wasn't a (bootable) "system disk" or it is a system disk, but it's broken.
Specifically, it's pathetic because it's grammatically ambiguous and requires additional knowledge.
It could easily be parsed grammatically as, "There was an error on the non-system disk or the disk" which makes no sense. Even when you parse it the way it was intended, you need to know that the PC tried to boot from a disk drive in order to comprehend the message.
"The program 'apt-get' is currently not installed. You can install it by typing: apt-get install apt"