That message pops up when you install from iso. This very confusing nonsense made me waste a lots of time. There was nothing wrong with medium at all, script just not found one file. It must be fixed to something sane, like"ERROR: Something is wrong with the installation medium! This is usually caused by burning tools which are not compatible with LMDE (YUMI or other multiboot tools). Please burn the ISO image to DVD/USB using a different tool."
."ERROR: not found filesystem.squashfs file in the given path"
And after all i suggest to give for the user a possibility to choose a path for installation source, to be able install from anywhere including network share, and only then check it availability, and of course before selecting language, user name, etc. And i think, that this will be not difficult to implement as scripts made of the plain text, can be easily edited and tested.